diff --git a/src/Symfony/Bridge/Doctrine/Tests/DataCollector/DoctrineDataCollectorTest.php b/src/Symfony/Bridge/Doctrine/Tests/DataCollector/DoctrineDataCollectorTest.php
index 6a33f0680a7a3..b9c9d51b713bc 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/DataCollector/DoctrineDataCollectorTest.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/DataCollector/DoctrineDataCollectorTest.php
@@ -205,7 +205,7 @@ private function createCollector($queries)
 
 class StringRepresentableClass
 {
-    public function __toString()
+    public function __toString(): string
     {
         return 'string representation';
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeIntIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeIntIdEntity.php
index 8a9b00ddc73e7..7c64cc20ad7e1 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeIntIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeIntIdEntity.php
@@ -34,7 +34,7 @@ public function __construct($id1, $id2, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->name;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeStringIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeStringIdEntity.php
index 0755a89e6a923..d6e8d2cd2aafa 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeStringIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/CompositeStringIdEntity.php
@@ -34,7 +34,7 @@ public function __construct($id1, $id2, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->name;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/Person.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/Person.php
index 6e383394bee47..b90a54ac02c61 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/Person.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/Person.php
@@ -38,7 +38,7 @@ public function __construct($id, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->name;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleAssociationToIntIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleAssociationToIntIdEntity.php
index 5cd6d407962aa..bed8bb9a51d01 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleAssociationToIntIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleAssociationToIntIdEntity.php
@@ -31,7 +31,7 @@ public function __construct(SingleIntIdNoToStringEntity $entity, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->name;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleIntIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleIntIdEntity.php
index ff29145e3353f..612566b45d94b 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleIntIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleIntIdEntity.php
@@ -33,7 +33,7 @@ public function __construct($id, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->name;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringCastableIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringCastableIdEntity.php
index e457f69dd091b..128801a02c922 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringCastableIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringCastableIdEntity.php
@@ -35,7 +35,7 @@ public function __construct($id, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->name;
     }
@@ -50,7 +50,7 @@ public function __construct($id)
         $this->id = $id;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->id;
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringIdEntity.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringIdEntity.php
index 3e25e2aea52bd..83f7a9f9ab39d 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringIdEntity.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/SingleStringIdEntity.php
@@ -30,7 +30,7 @@ public function __construct($id, $name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->name;
     }
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/app/AppKernel.php b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/app/AppKernel.php
index 0ea364b196940..0d329582b39ef 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/app/AppKernel.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/app/AppKernel.php
@@ -79,7 +79,7 @@ protected function build(ContainerBuilder $container)
         $container->register('logger', NullLogger::class);
     }
 
-    public function __sleep()
+    public function __sleep(): array
     {
         return ['varDir', 'testCase', 'rootConfig', 'environment', 'debug'];
     }
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/Kernel/ConcreteMicroKernel.php b/src/Symfony/Bundle/FrameworkBundle/Tests/Kernel/ConcreteMicroKernel.php
index 7e6bb3481064c..954eef5948eed 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/Kernel/ConcreteMicroKernel.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/Kernel/ConcreteMicroKernel.php
@@ -64,7 +64,7 @@ public function getLogDir(): string
         return $this->cacheDir;
     }
 
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }
diff --git a/src/Symfony/Bundle/TwigBundle/TemplateIterator.php b/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
index b96a18318bf48..2f33a9ed310fb 100644
--- a/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
+++ b/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
@@ -43,7 +43,7 @@ public function __construct(KernelInterface $kernel, string $rootDir, array $pat
     }
 
     /**
-     * {@inheritdoc}
+     * @return \Traversable
      */
     public function getIterator()
     {
diff --git a/src/Symfony/Component/Config/Resource/GlobResource.php b/src/Symfony/Component/Config/Resource/GlobResource.php
index e33cafc6e0266..ad7531dcd5c70 100644
--- a/src/Symfony/Component/Config/Resource/GlobResource.php
+++ b/src/Symfony/Component/Config/Resource/GlobResource.php
@@ -91,6 +91,9 @@ public function __sleep(): array
         return ['prefix', 'pattern', 'recursive', 'hash', 'forExclusion', 'excludedPrefixes'];
     }
 
+    /**
+     * @return \Traversable
+     */
     public function getIterator()
     {
         if (!file_exists($this->prefix) || (!$this->recursive && '' === $this->pattern)) {
diff --git a/src/Symfony/Component/Console/Helper/TableRows.php b/src/Symfony/Component/Console/Helper/TableRows.php
index 4809daf1cac80..16aabb3fc9350 100644
--- a/src/Symfony/Component/Console/Helper/TableRows.php
+++ b/src/Symfony/Component/Console/Helper/TableRows.php
@@ -23,7 +23,7 @@ public function __construct(callable $generator)
         $this->generator = $generator;
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         $g = $this->generator;
 
diff --git a/src/Symfony/Component/Console/Tests/Formatter/OutputFormatterTest.php b/src/Symfony/Component/Console/Tests/Formatter/OutputFormatterTest.php
index 440088822bdb5..1bd2b5d57bdf2 100644
--- a/src/Symfony/Component/Console/Tests/Formatter/OutputFormatterTest.php
+++ b/src/Symfony/Component/Console/Tests/Formatter/OutputFormatterTest.php
@@ -339,7 +339,7 @@ public function testFormatAndWrap()
 
 class TableCell
 {
-    public function __toString()
+    public function __toString(): string
     {
         return '<info>some info</info>';
     }
diff --git a/src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php b/src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php
index f9122a74c8536..83d01f6818517 100644
--- a/src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php
+++ b/src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php
@@ -835,7 +835,7 @@ public function __construct(array $values)
         $this->values = $values;
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->values);
     }
diff --git a/src/Symfony/Component/Console/Tests/Logger/ConsoleLoggerTest.php b/src/Symfony/Component/Console/Tests/Logger/ConsoleLoggerTest.php
index ef115767efccb..0f5884136b021 100644
--- a/src/Symfony/Component/Console/Tests/Logger/ConsoleLoggerTest.php
+++ b/src/Symfony/Component/Console/Tests/Logger/ConsoleLoggerTest.php
@@ -204,7 +204,7 @@ public function testContextExceptionKeyCanBeExceptionOrOtherValues()
 
 class DummyTest
 {
-    public function __toString()
+    public function __toString(): string
     {
     }
 }
diff --git a/src/Symfony/Component/DependencyInjection/Argument/RewindableGenerator.php b/src/Symfony/Component/DependencyInjection/Argument/RewindableGenerator.php
index b00a36c34f542..41fec786fd739 100644
--- a/src/Symfony/Component/DependencyInjection/Argument/RewindableGenerator.php
+++ b/src/Symfony/Component/DependencyInjection/Argument/RewindableGenerator.php
@@ -28,14 +28,14 @@ public function __construct(callable $generator, $count)
         $this->count = $count;
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         $g = $this->generator;
 
         return $g();
     }
 
-    public function count()
+    public function count(): int
     {
         if (\is_callable($count = $this->count)) {
             $this->count = $count();
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectExtension.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectExtension.php
index 81ea2b18bb4fc..4dee6add01911 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectExtension.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectExtension.php
@@ -25,17 +25,17 @@ public function load(array $configs, ContainerBuilder $configuration)
         return $configuration;
     }
 
-    public function getXsdValidationBasePath()
+    public function getXsdValidationBasePath(): string
     {
         return false;
     }
 
-    public function getNamespace()
+    public function getNamespace(): string
     {
         return 'http://www.example.com/schema/project';
     }
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return 'project';
     }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtension.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtension.php
index 2ee2f12dc8f58..f457abd2a379d 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtension.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtension.php
@@ -2,17 +2,17 @@
 
 class ProjectWithXsdExtension extends ProjectExtension
 {
-    public function getXsdValidationBasePath()
+    public function getXsdValidationBasePath(): string
     {
         return __DIR__.'/schema';
     }
 
-    public function getNamespace()
+    public function getNamespace(): string
     {
         return 'http://www.example.com/schema/projectwithxsd';
     }
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return 'projectwithxsd';
     }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtensionInPhar.phar b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtensionInPhar.phar
index 040e136a34252..93d4e87c0b89b 100644
Binary files a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtensionInPhar.phar and b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/ProjectWithXsdExtensionInPhar.phar differ
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/classes.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/classes.php
index 29913a8556093..f3a490691a41d 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/classes.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/classes.php
@@ -83,17 +83,17 @@ public function callPassed()
 
 class DummyProxyDumper implements ProxyDumper
 {
-    public function isProxyCandidate(Definition $definition)
+    public function isProxyCandidate(Definition $definition): bool
     {
         return $definition->isLazy();
     }
 
-    public function getProxyFactoryCode(Definition $definition, $id, $factoryCall = null)
+    public function getProxyFactoryCode(Definition $definition, $id, $factoryCall = null): string
     {
         return "        // lazy factory for {$definition->getClass()}\n\n";
     }
 
-    public function getProxyCode(Definition $definition)
+    public function getProxyCode(Definition $definition): string
     {
         return "// proxy code for {$definition->getClass()}\n";
     }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/createphar.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/createphar.php
index c675478fd639c..3ad4a8cf0e04e 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/createphar.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/createphar.php
@@ -11,17 +11,17 @@
 
 class ProjectWithXsdExtensionInPhar extends ProjectExtension
 {
-    public function getXsdValidationBasePath()
+    public function getXsdValidationBasePath(): string
     {
         return __DIR__.'/schema';
     }
 
-    public function getNamespace()
+    public function getNamespace(): string
     {
         return 'http://www.example.com/schema/projectwithxsdinphar';
     }
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return 'projectwithxsdinphar';
     }
diff --git a/src/Symfony/Component/Finder/Iterator/ExcludeDirectoryFilterIterator.php b/src/Symfony/Component/Finder/Iterator/ExcludeDirectoryFilterIterator.php
index eb37f1310e428..6a1b291adea30 100644
--- a/src/Symfony/Component/Finder/Iterator/ExcludeDirectoryFilterIterator.php
+++ b/src/Symfony/Component/Finder/Iterator/ExcludeDirectoryFilterIterator.php
@@ -68,6 +68,9 @@ public function accept()
         return true;
     }
 
+    /**
+     * @return bool
+     */
     public function hasChildren()
     {
         return $this->isRecursive && $this->iterator->hasChildren();
diff --git a/src/Symfony/Component/Finder/Iterator/SortableIterator.php b/src/Symfony/Component/Finder/Iterator/SortableIterator.php
index eda093fa2ce22..cc955a7108b36 100644
--- a/src/Symfony/Component/Finder/Iterator/SortableIterator.php
+++ b/src/Symfony/Component/Finder/Iterator/SortableIterator.php
@@ -80,6 +80,9 @@ public function __construct(\Traversable $iterator, $sort, bool $reverseOrder =
         }
     }
 
+    /**
+     * @return \Traversable
+     */
     public function getIterator()
     {
         if (1 === $this->sort) {
diff --git a/src/Symfony/Component/Finder/Tests/Iterator/Iterator.php b/src/Symfony/Component/Finder/Tests/Iterator/Iterator.php
index 3e21a070047aa..bc2eb53b393e0 100644
--- a/src/Symfony/Component/Finder/Tests/Iterator/Iterator.php
+++ b/src/Symfony/Component/Finder/Tests/Iterator/Iterator.php
@@ -33,7 +33,7 @@ public function rewind()
         reset($this->values);
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return false !== $this->current();
     }
diff --git a/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php b/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php
index df7726a83f019..ee600878f2a30 100644
--- a/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php
+++ b/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php
@@ -234,7 +234,7 @@ public function getData()
     /**
      * @internal
      */
-    public function __sleep()
+    public function __sleep(): array
     {
         foreach ($this->data['forms_by_hash'] as &$form) {
             if (isset($form['type_class']) && !$form['type_class'] instanceof ClassStub) {
diff --git a/src/Symfony/Component/Form/FormBuilder.php b/src/Symfony/Component/Form/FormBuilder.php
index 87b4485a6f11b..e9c7213a98944 100644
--- a/src/Symfony/Component/Form/FormBuilder.php
+++ b/src/Symfony/Component/Form/FormBuilder.php
@@ -158,7 +158,7 @@ public function all()
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function count()
     {
diff --git a/src/Symfony/Component/Form/Tests/ChoiceList/Factory/DefaultChoiceListFactoryTest.php b/src/Symfony/Component/Form/Tests/ChoiceList/Factory/DefaultChoiceListFactoryTest.php
index b39f67acb7522..4472bd06c9403 100644
--- a/src/Symfony/Component/Form/Tests/ChoiceList/Factory/DefaultChoiceListFactoryTest.php
+++ b/src/Symfony/Component/Form/Tests/ChoiceList/Factory/DefaultChoiceListFactoryTest.php
@@ -860,7 +860,7 @@ public function __construct($property)
         $this->property = $property;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->property;
     }
diff --git a/src/Symfony/Component/Form/Tests/Fixtures/CustomArrayObject.php b/src/Symfony/Component/Form/Tests/Fixtures/CustomArrayObject.php
index 47fa14090e26d..5c12b6b400bb8 100644
--- a/src/Symfony/Component/Form/Tests/Fixtures/CustomArrayObject.php
+++ b/src/Symfony/Component/Form/Tests/Fixtures/CustomArrayObject.php
@@ -24,7 +24,7 @@ public function __construct(array $array = null)
         $this->array = $array ?: [];
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->array);
     }
@@ -48,12 +48,12 @@ public function offsetUnset($offset)
         unset($this->array[$offset]);
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->array);
     }
 
-    public function count()
+    public function count(): int
     {
         return \count($this->array);
     }
@@ -63,7 +63,7 @@ public function __serialize(): array
         return $this->array;
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         return serialize($this->__serialize());
     }
diff --git a/src/Symfony/Component/Form/Tests/SimpleFormTest.php b/src/Symfony/Component/Form/Tests/SimpleFormTest.php
index 33f815891b3fc..949885222e910 100644
--- a/src/Symfony/Component/Form/Tests/SimpleFormTest.php
+++ b/src/Symfony/Component/Form/Tests/SimpleFormTest.php
@@ -32,7 +32,7 @@ public function __construct($count)
         $this->count = $count;
     }
 
-    public function count()
+    public function count(): int
     {
         return $this->count;
     }
@@ -47,7 +47,7 @@ public function __construct($count)
         $this->iterator = new \ArrayIterator($count > 0 ? array_fill(0, $count, 'Foo') : []);
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return $this->iterator;
     }
diff --git a/src/Symfony/Component/Form/Util/InheritDataAwareIterator.php b/src/Symfony/Component/Form/Util/InheritDataAwareIterator.php
index b48549c6e03a4..ec937570593c6 100644
--- a/src/Symfony/Component/Form/Util/InheritDataAwareIterator.php
+++ b/src/Symfony/Component/Form/Util/InheritDataAwareIterator.php
@@ -34,7 +34,7 @@ public function getChildren()
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function hasChildren()
     {
diff --git a/src/Symfony/Component/Form/Util/OrderedHashMap.php b/src/Symfony/Component/Form/Util/OrderedHashMap.php
index 19ec4fc7d2a27..5d67b3124ec72 100644
--- a/src/Symfony/Component/Form/Util/OrderedHashMap.php
+++ b/src/Symfony/Component/Form/Util/OrderedHashMap.php
@@ -99,7 +99,7 @@ public function __construct(array $elements = [])
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function offsetExists($key)
     {
@@ -157,7 +157,7 @@ public function offsetUnset($key)
     }
 
     /**
-     * {@inheritdoc}
+     * @return \Traversable
      */
     public function getIterator()
     {
@@ -165,7 +165,7 @@ public function getIterator()
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function count()
     {
diff --git a/src/Symfony/Component/Form/Util/OrderedHashMapIterator.php b/src/Symfony/Component/Form/Util/OrderedHashMapIterator.php
index c34bf2e39b134..323fdd232961a 100644
--- a/src/Symfony/Component/Form/Util/OrderedHashMapIterator.php
+++ b/src/Symfony/Component/Form/Util/OrderedHashMapIterator.php
@@ -128,7 +128,7 @@ public function key()
     /**
      * {@inheritdoc}
      */
-    public function valid()
+    public function valid(): bool
     {
         return null !== $this->key;
     }
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/AbstractSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/AbstractSessionHandler.php
index 78340efbd2228..bcde59ee6b6d8 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/AbstractSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/AbstractSessionHandler.php
@@ -29,7 +29,7 @@ abstract class AbstractSessionHandler implements \SessionHandlerInterface, \Sess
     private $igbinaryEmptyData;
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function open($savePath, $sessionName)
     {
@@ -64,7 +64,7 @@ abstract protected function doWrite($sessionId, $data);
     abstract protected function doDestroy($sessionId);
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function validateId($sessionId)
     {
@@ -75,7 +75,7 @@ public function validateId($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return string
      */
     public function read($sessionId)
     {
@@ -98,7 +98,7 @@ public function read($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function write($sessionId, $data)
     {
@@ -115,7 +115,7 @@ public function write($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function destroy($sessionId)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MemcachedSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MemcachedSessionHandler.php
index 3faa336f69dd6..7a528e655a291 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MemcachedSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MemcachedSessionHandler.php
@@ -55,7 +55,7 @@ public function __construct(\Memcached $memcached, array $options = [])
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -71,7 +71,7 @@ protected function doRead($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
@@ -99,7 +99,7 @@ protected function doDestroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MigratingSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MigratingSessionHandler.php
index 5293d2448a29e..ccf23de7eb299 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MigratingSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MigratingSessionHandler.php
@@ -39,7 +39,7 @@ public function __construct(\SessionHandlerInterface $currentHandler, \SessionHa
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -50,7 +50,7 @@ public function close()
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function destroy($sessionId)
     {
@@ -61,7 +61,7 @@ public function destroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
@@ -72,7 +72,7 @@ public function gc($maxlifetime)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function open($savePath, $sessionName)
     {
@@ -83,7 +83,7 @@ public function open($savePath, $sessionName)
     }
 
     /**
-     * {@inheritdoc}
+     * @return string
      */
     public function read($sessionId)
     {
@@ -92,7 +92,7 @@ public function read($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function write($sessionId, $sessionData)
     {
@@ -103,7 +103,7 @@ public function write($sessionId, $sessionData)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function validateId($sessionId)
     {
@@ -112,7 +112,7 @@ public function validateId($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $sessionData)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
index 53d9369e8d53b..ad1bd24d4ff90 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
@@ -80,7 +80,7 @@ public function __construct(\MongoDB\Client $mongo, array $options)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -100,7 +100,7 @@ protected function doDestroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
@@ -134,7 +134,7 @@ protected function doWrite($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NullSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NullSessionHandler.php
index 8d193155b090f..ba063960469c0 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NullSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NullSessionHandler.php
@@ -19,7 +19,7 @@
 class NullSessionHandler extends AbstractSessionHandler
 {
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -27,7 +27,7 @@ public function close()
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function validateId($sessionId)
     {
@@ -43,7 +43,7 @@ protected function doRead($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
@@ -67,7 +67,7 @@ protected function doDestroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php
index 043252cd31885..0cd1296c09169 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php
@@ -260,7 +260,7 @@ public function isSessionExpired()
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function open($savePath, $sessionName)
     {
@@ -274,7 +274,7 @@ public function open($savePath, $sessionName)
     }
 
     /**
-     * {@inheritdoc}
+     * @return string
      */
     public function read($sessionId)
     {
@@ -288,7 +288,7 @@ public function read($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
@@ -367,7 +367,7 @@ protected function doWrite($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
@@ -391,7 +391,7 @@ public function updateTimestamp($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/RedisSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/RedisSessionHandler.php
index c2e7d34dcfe09..752e58f73f58a 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/RedisSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/RedisSessionHandler.php
@@ -104,7 +104,7 @@ public function gc($maxlifetime): bool
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/StrictSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/StrictSessionHandler.php
index 83a1f2c063c05..31a39a6973a42 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/StrictSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/StrictSessionHandler.php
@@ -31,7 +31,7 @@ public function __construct(\SessionHandlerInterface $handler)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function open($savePath, $sessionName)
     {
@@ -49,7 +49,7 @@ protected function doRead($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
@@ -65,7 +65,7 @@ protected function doWrite($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function destroy($sessionId)
     {
@@ -86,7 +86,7 @@ protected function doDestroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -94,7 +94,7 @@ public function close()
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Proxy/SessionHandlerProxy.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Proxy/SessionHandlerProxy.php
index b11cc397a0973..8034b7a46b1e2 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Proxy/SessionHandlerProxy.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Proxy/SessionHandlerProxy.php
@@ -36,7 +36,7 @@ public function getHandler()
     // \SessionHandlerInterface
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function open($savePath, $sessionName)
     {
@@ -44,7 +44,7 @@ public function open($savePath, $sessionName)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function close()
     {
@@ -52,7 +52,7 @@ public function close()
     }
 
     /**
-     * {@inheritdoc}
+     * @return string
      */
     public function read($sessionId)
     {
@@ -60,7 +60,7 @@ public function read($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function write($sessionId, $data)
     {
@@ -68,7 +68,7 @@ public function write($sessionId, $data)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function destroy($sessionId)
     {
@@ -76,7 +76,7 @@ public function destroy($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function gc($maxlifetime)
     {
@@ -84,7 +84,7 @@ public function gc($maxlifetime)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function validateId($sessionId)
     {
@@ -92,7 +92,7 @@ public function validateId($sessionId)
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function updateTimestamp($sessionId, $data)
     {
diff --git a/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php b/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
index 03b29653e8831..edce7dbf4bbfe 100644
--- a/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
+++ b/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
@@ -1054,7 +1054,7 @@ public function testReasonPhraseDefaultsAgainstIana($code, $reasonPhrase)
 
 class StringableObject
 {
-    public function __toString()
+    public function __toString(): string
     {
         return 'Foo';
     }
diff --git a/src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php b/src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php
index 408d6d31a21e8..7ace03ae68a5c 100644
--- a/src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php
+++ b/src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php
@@ -148,7 +148,7 @@ public function reset()
     /**
      * @internal
      */
-    public function __sleep()
+    public function __sleep(): array
     {
         if (!$this->dataCount) {
             $this->data = [];
diff --git a/src/Symfony/Component/HttpKernel/Tests/Controller/ControllerResolverTest.php b/src/Symfony/Component/HttpKernel/Tests/Controller/ControllerResolverTest.php
index 77ce524fc60a2..014b21e83aff3 100644
--- a/src/Symfony/Component/HttpKernel/Tests/Controller/ControllerResolverTest.php
+++ b/src/Symfony/Component/HttpKernel/Tests/Controller/ControllerResolverTest.php
@@ -203,7 +203,7 @@ public function __construct()
     {
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return '';
     }
diff --git a/src/Symfony/Component/HttpKernel/Tests/KernelTest.php b/src/Symfony/Component/HttpKernel/Tests/KernelTest.php
index 35304bc982ef1..3d82538279633 100644
--- a/src/Symfony/Component/HttpKernel/Tests/KernelTest.php
+++ b/src/Symfony/Component/HttpKernel/Tests/KernelTest.php
@@ -21,7 +21,6 @@
 use Symfony\Component\HttpKernel\Bundle\BundleInterface;
 use Symfony\Component\HttpKernel\DependencyInjection\ResettableServicePass;
 use Symfony\Component\HttpKernel\DependencyInjection\ServicesResetter;
-use Symfony\Component\HttpKernel\HttpKernel;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\HttpKernel\Kernel;
 use Symfony\Component\HttpKernel\Tests\Fixtures\KernelForOverrideName;
diff --git a/src/Symfony/Component/HttpKernel/Tests/Log/LoggerTest.php b/src/Symfony/Component/HttpKernel/Tests/Log/LoggerTest.php
index c89146aff5c12..c562af1bcb7a6 100644
--- a/src/Symfony/Component/HttpKernel/Tests/Log/LoggerTest.php
+++ b/src/Symfony/Component/HttpKernel/Tests/Log/LoggerTest.php
@@ -200,7 +200,7 @@ public function testFormatter()
 
 class DummyTest
 {
-    public function __toString()
+    public function __toString(): string
     {
     }
 }
diff --git a/src/Symfony/Component/Intl/Data/Util/ArrayAccessibleResourceBundle.php b/src/Symfony/Component/Intl/Data/Util/ArrayAccessibleResourceBundle.php
index d1de8d5b096e7..037f704df7064 100644
--- a/src/Symfony/Component/Intl/Data/Util/ArrayAccessibleResourceBundle.php
+++ b/src/Symfony/Component/Intl/Data/Util/ArrayAccessibleResourceBundle.php
@@ -39,7 +39,7 @@ public function get($offset)
         return $value instanceof \ResourceBundle ? new static($value) : $value;
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return null !== $this->bundleImpl->get($offset);
     }
@@ -59,12 +59,12 @@ public function offsetUnset($offset)
         throw new BadMethodCallException('Resource bundles cannot be modified.');
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return $this->bundleImpl;
     }
 
-    public function count()
+    public function count(): int
     {
         return $this->bundleImpl->count();
     }
diff --git a/src/Symfony/Component/Intl/Data/Util/RingBuffer.php b/src/Symfony/Component/Intl/Data/Util/RingBuffer.php
index 9b4a8605942f3..db33a4fc1599b 100644
--- a/src/Symfony/Component/Intl/Data/Util/RingBuffer.php
+++ b/src/Symfony/Component/Intl/Data/Util/RingBuffer.php
@@ -42,7 +42,7 @@ public function __construct(int $size)
     /**
      * {@inheritdoc}
      */
-    public function offsetExists($key)
+    public function offsetExists($key): bool
     {
         return isset($this->indices[$key]);
     }
diff --git a/src/Symfony/Component/Ldap/Adapter/ExtLdap/Collection.php b/src/Symfony/Component/Ldap/Adapter/ExtLdap/Collection.php
index 4aa9663dc9930..2f3d22743d22d 100644
--- a/src/Symfony/Component/Ldap/Adapter/ExtLdap/Collection.php
+++ b/src/Symfony/Component/Ldap/Adapter/ExtLdap/Collection.php
@@ -42,6 +42,9 @@ public function toArray()
         return $this->entries;
     }
 
+    /**
+     * @return int
+     */
     public function count()
     {
         $con = $this->connection->getResource();
@@ -58,6 +61,9 @@ public function count()
         return $count;
     }
 
+    /**
+     * @return \Traversable
+     */
     public function getIterator()
     {
         if (0 === $this->count()) {
@@ -81,6 +87,9 @@ public function getIterator()
         }
     }
 
+    /**
+     * @return bool
+     */
     public function offsetExists($offset)
     {
         $this->toArray();
diff --git a/src/Symfony/Component/Ldap/Tests/LdapTest.php b/src/Symfony/Component/Ldap/Tests/LdapTest.php
index 68e9da2451131..881a15a4b29e0 100644
--- a/src/Symfony/Component/Ldap/Tests/LdapTest.php
+++ b/src/Symfony/Component/Ldap/Tests/LdapTest.php
@@ -55,8 +55,9 @@ public function testLdapEscape()
             ->expects($this->once())
             ->method('escape')
             ->with('foo', 'bar', 'baz')
-            ->willReturn('');
+            ->willReturn('')
         ;
+
         $this->ldap->escape('foo', 'bar', 'baz');
     }
 
diff --git a/src/Symfony/Component/Lock/Key.php b/src/Symfony/Component/Lock/Key.php
index 798e571d008da..c53892ab6bed2 100644
--- a/src/Symfony/Component/Lock/Key.php
+++ b/src/Symfony/Component/Lock/Key.php
@@ -27,7 +27,7 @@ public function __construct(string $resource)
         $this->resource = $resource;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->resource;
     }
diff --git a/src/Symfony/Component/Mime/RawMessage.php b/src/Symfony/Component/Mime/RawMessage.php
index 17501ec1d0217..8a01049e51301 100644
--- a/src/Symfony/Component/Mime/RawMessage.php
+++ b/src/Symfony/Component/Mime/RawMessage.php
@@ -54,7 +54,7 @@ public function toIterable(): iterable
     /**
      * @internal
      */
-    final public function serialize()
+    final public function serialize(): string
     {
         return serialize($this->__serialize());
     }
diff --git a/src/Symfony/Component/Process/InputStream.php b/src/Symfony/Component/Process/InputStream.php
index 426ffa33a35aa..c952daf592f07 100644
--- a/src/Symfony/Component/Process/InputStream.php
+++ b/src/Symfony/Component/Process/InputStream.php
@@ -66,6 +66,9 @@ public function isClosed()
         return !$this->open;
     }
 
+    /**
+     * @return \Traversable
+     */
     public function getIterator()
     {
         $this->open = true;
diff --git a/src/Symfony/Component/PropertyAccess/Tests/Fixtures/NonTraversableArrayObject.php b/src/Symfony/Component/PropertyAccess/Tests/Fixtures/NonTraversableArrayObject.php
index cb659f907c15b..cf02ee69f2979 100644
--- a/src/Symfony/Component/PropertyAccess/Tests/Fixtures/NonTraversableArrayObject.php
+++ b/src/Symfony/Component/PropertyAccess/Tests/Fixtures/NonTraversableArrayObject.php
@@ -24,7 +24,7 @@ public function __construct(array $array = null)
         $this->array = $array ?: [];
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->array);
     }
@@ -48,7 +48,7 @@ public function offsetUnset($offset)
         unset($this->array[$offset]);
     }
 
-    public function count()
+    public function count(): int
     {
         return \count($this->array);
     }
@@ -58,7 +58,7 @@ public function __serialize(): array
         return $this->array;
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         return serialize($this->__serialize());
     }
diff --git a/src/Symfony/Component/PropertyAccess/Tests/Fixtures/TraversableArrayObject.php b/src/Symfony/Component/PropertyAccess/Tests/Fixtures/TraversableArrayObject.php
index ba5ec36e76bd2..5693c6b73e685 100644
--- a/src/Symfony/Component/PropertyAccess/Tests/Fixtures/TraversableArrayObject.php
+++ b/src/Symfony/Component/PropertyAccess/Tests/Fixtures/TraversableArrayObject.php
@@ -24,7 +24,7 @@ public function __construct(array $array = null)
         $this->array = $array ?: [];
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->array);
     }
@@ -48,12 +48,12 @@ public function offsetUnset($offset)
         unset($this->array[$offset]);
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->array);
     }
 
-    public function count()
+    public function count(): int
     {
         return \count($this->array);
     }
@@ -63,7 +63,7 @@ public function __serialize(): array
         return $this->array;
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         return serialize($this->__serialize());
     }
diff --git a/src/Symfony/Component/Routing/CompiledRoute.php b/src/Symfony/Component/Routing/CompiledRoute.php
index 87278e702a899..b6f31b2ebe94d 100644
--- a/src/Symfony/Component/Routing/CompiledRoute.php
+++ b/src/Symfony/Component/Routing/CompiledRoute.php
@@ -64,6 +64,8 @@ public function __serialize(): array
     }
 
     /**
+     * @return string
+     *
      * @internal since Symfony 4.3
      * @final since Symfony 4.3
      */
diff --git a/src/Symfony/Component/Routing/Route.php b/src/Symfony/Component/Routing/Route.php
index b402665b9220a..03ec76e0dde26 100644
--- a/src/Symfony/Component/Routing/Route.php
+++ b/src/Symfony/Component/Routing/Route.php
@@ -78,6 +78,8 @@ public function __serialize(): array
     }
 
     /**
+     * @return string
+     *
      * @internal since Symfony 4.3
      * @final since Symfony 4.3
      */
diff --git a/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php b/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
index ce16144639856..6c9903c780227 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
@@ -163,10 +163,9 @@ public function __serialize(): array
     }
 
     /**
-     * {@inheritdoc}
+     * @return string
      *
      * @final since Symfony 4.3, use __serialize() instead
-     *
      * @internal since Symfony 4.3, use __serialize() instead
      */
     public function serialize()
diff --git a/src/Symfony/Component/Security/Core/Exception/AuthenticationException.php b/src/Symfony/Component/Security/Core/Exception/AuthenticationException.php
index 94e8d24cb7106..1cfec5352f31d 100644
--- a/src/Symfony/Component/Security/Core/Exception/AuthenticationException.php
+++ b/src/Symfony/Component/Security/Core/Exception/AuthenticationException.php
@@ -113,7 +113,7 @@ public function unserialize($serialized)
     /**
      * @internal
      */
-    public function __sleep()
+    public function __sleep(): array
     {
         if (__CLASS__ !== $c = (new \ReflectionMethod($this, 'serialize'))->getDeclaringClass()->name) {
             @trigger_error(sprintf('Implementing the "%s::serialize()" method is deprecated since Symfony 4.3, implement the __serialize() and __unserialize() methods instead.', $c), E_USER_DEPRECATED);
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
index 3556f4afeafcc..a446e61602be8 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
@@ -152,7 +152,7 @@ public function __serialize(): array
     {
     }
 
-    public function serialize()
+    public function serialize(): string
     {
     }
 
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
index c8b5fed8d77f3..fe0ed08cc66d2 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
@@ -259,7 +259,7 @@ public function __construct($name)
         $this->name = $name;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->name;
     }
diff --git a/src/Symfony/Component/Security/Core/Tests/SecurityTest.php b/src/Symfony/Component/Security/Core/Tests/SecurityTest.php
index e30b14e7f97a9..49f6f8dbe3c3d 100644
--- a/src/Symfony/Component/Security/Core/Tests/SecurityTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/SecurityTest.php
@@ -122,7 +122,7 @@ private function createContainer($serviceId, $serviceObject)
 
 class StringishUser
 {
-    public function __toString()
+    public function __toString(): string
     {
         return 'stringish_user';
     }
diff --git a/src/Symfony/Component/Security/Core/User/User.php b/src/Symfony/Component/Security/Core/User/User.php
index cd763821c8391..ce41c02a1c7d8 100644
--- a/src/Symfony/Component/Security/Core/User/User.php
+++ b/src/Symfony/Component/Security/Core/User/User.php
@@ -45,7 +45,7 @@ public function __construct(?string $username, ?string $password, array $roles =
         $this->extraFields = $extraFields;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->getUsername();
     }
diff --git a/src/Symfony/Component/Security/Guard/Tests/Authenticator/FormLoginAuthenticatorTest.php b/src/Symfony/Component/Security/Guard/Tests/Authenticator/FormLoginAuthenticatorTest.php
index 56ee7f39197b5..e0cd29131e125 100644
--- a/src/Symfony/Component/Security/Guard/Tests/Authenticator/FormLoginAuthenticatorTest.php
+++ b/src/Symfony/Component/Security/Guard/Tests/Authenticator/FormLoginAuthenticatorTest.php
@@ -111,7 +111,7 @@ public function onAuthenticationSuccess(Request $request, TokenInterface $token,
     /**
      * @param mixed $defaultSuccessRedirectUrl
      */
-    public function setDefaultSuccessRedirectUrl($defaultSuccessRedirectUrl): TestFormLoginAuthenticator
+    public function setDefaultSuccessRedirectUrl($defaultSuccessRedirectUrl): self
     {
         $this->defaultSuccessRedirectUrl = $defaultSuccessRedirectUrl;
 
@@ -121,7 +121,7 @@ public function setDefaultSuccessRedirectUrl($defaultSuccessRedirectUrl): TestFo
     /**
      * @param mixed $loginUrl
      */
-    public function setLoginUrl($loginUrl): TestFormLoginAuthenticator
+    public function setLoginUrl($loginUrl): self
     {
         $this->loginUrl = $loginUrl;
 
diff --git a/src/Symfony/Component/Security/Http/Tests/Firewall/UsernamePasswordFormAuthenticationListenerTest.php b/src/Symfony/Component/Security/Http/Tests/Firewall/UsernamePasswordFormAuthenticationListenerTest.php
index 723b4ebb97cff..c83d7e878713a 100644
--- a/src/Symfony/Component/Security/Http/Tests/Firewall/UsernamePasswordFormAuthenticationListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Firewall/UsernamePasswordFormAuthenticationListenerTest.php
@@ -191,7 +191,7 @@ public function getUsernameForLength()
 
 class DummyUserClass
 {
-    public function __toString()
+    public function __toString(): string
     {
         return '';
     }
diff --git a/src/Symfony/Component/Serializer/Tests/Fixtures/TraversableDummy.php b/src/Symfony/Component/Serializer/Tests/Fixtures/TraversableDummy.php
index bcf46e512e26a..ac98f1c2e76e6 100644
--- a/src/Symfony/Component/Serializer/Tests/Fixtures/TraversableDummy.php
+++ b/src/Symfony/Component/Serializer/Tests/Fixtures/TraversableDummy.php
@@ -16,7 +16,7 @@ class TraversableDummy implements \IteratorAggregate
     public $foo = 'foo';
     public $bar = 'bar';
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator(get_object_vars($this));
     }
diff --git a/src/Symfony/Component/Templating/Tests/Fixtures/SimpleHelper.php b/src/Symfony/Component/Templating/Tests/Fixtures/SimpleHelper.php
index 9538fcc313b54..5ddb9f13a9399 100644
--- a/src/Symfony/Component/Templating/Tests/Fixtures/SimpleHelper.php
+++ b/src/Symfony/Component/Templating/Tests/Fixtures/SimpleHelper.php
@@ -22,7 +22,7 @@ public function __construct($value)
         $this->value = $value;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->value;
     }
diff --git a/src/Symfony/Component/Translation/Tests/TranslatorTest.php b/src/Symfony/Component/Translation/Tests/TranslatorTest.php
index 87603a9dc867b..f171ed93fc812 100644
--- a/src/Symfony/Component/Translation/Tests/TranslatorTest.php
+++ b/src/Symfony/Component/Translation/Tests/TranslatorTest.php
@@ -662,7 +662,7 @@ public function __construct($str)
         $this->str = $str;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->str;
     }
diff --git a/src/Symfony/Component/Validator/ConstraintViolationList.php b/src/Symfony/Component/Validator/ConstraintViolationList.php
index c30ee57cb268b..e56c8e31f1253 100644
--- a/src/Symfony/Component/Validator/ConstraintViolationList.php
+++ b/src/Symfony/Component/Validator/ConstraintViolationList.php
@@ -116,7 +116,7 @@ public function getIterator()
     }
 
     /**
-     * {@inheritdoc}
+     * @return int
      */
     public function count()
     {
@@ -124,7 +124,7 @@ public function count()
     }
 
     /**
-     * {@inheritdoc}
+     * @return bool
      */
     public function offsetExists($offset)
     {
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/AbstractComparisonValidatorTestCase.php b/src/Symfony/Component/Validator/Tests/Constraints/AbstractComparisonValidatorTestCase.php
index 486901bfc31c9..944195f0f35bc 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/AbstractComparisonValidatorTestCase.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/AbstractComparisonValidatorTestCase.php
@@ -25,7 +25,7 @@ public function __construct($value)
         $this->value = $value;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->value;
     }
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/BicValidatorTest.php b/src/Symfony/Component/Validator/Tests/Constraints/BicValidatorTest.php
index 80c00423e7745..b07f6f9f83b26 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/BicValidatorTest.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/BicValidatorTest.php
@@ -248,7 +248,7 @@ public function __construct($value)
         $this->value = $value;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return (string) $this->value;
     }
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/UrlValidatorTest.php b/src/Symfony/Component/Validator/Tests/Constraints/UrlValidatorTest.php
index e59c6fe5c07f5..bcfa07ad9af4e 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/UrlValidatorTest.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/UrlValidatorTest.php
@@ -387,7 +387,7 @@ public function testCheckDnsOptionIsDeprecated()
 
 class EmailProvider
 {
-    public function __toString()
+    public function __toString(): string
     {
         return '';
     }
diff --git a/src/Symfony/Component/Validator/Tests/Fixtures/Countable.php b/src/Symfony/Component/Validator/Tests/Fixtures/Countable.php
index afc42376a254b..6479c23b2dd2c 100644
--- a/src/Symfony/Component/Validator/Tests/Fixtures/Countable.php
+++ b/src/Symfony/Component/Validator/Tests/Fixtures/Countable.php
@@ -20,7 +20,7 @@ public function __construct(array $content)
         $this->content = $content;
     }
 
-    public function count()
+    public function count(): int
     {
         return \count($this->content);
     }
diff --git a/src/Symfony/Component/Validator/Tests/Fixtures/CustomArrayObject.php b/src/Symfony/Component/Validator/Tests/Fixtures/CustomArrayObject.php
index 9b5303c167c0b..34b208b2bea0c 100644
--- a/src/Symfony/Component/Validator/Tests/Fixtures/CustomArrayObject.php
+++ b/src/Symfony/Component/Validator/Tests/Fixtures/CustomArrayObject.php
@@ -24,7 +24,7 @@ public function __construct(array $array = null)
         $this->array = $array ?: [];
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->array);
     }
@@ -48,12 +48,12 @@ public function offsetUnset($offset)
         unset($this->array[$offset]);
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->array);
     }
 
-    public function count()
+    public function count(): int
     {
         return \count($this->array);
     }
@@ -63,7 +63,7 @@ public function __serialize(): array
         return $this->array;
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         return serialize($this->__serialize());
     }
diff --git a/src/Symfony/Component/Validator/Tests/Fixtures/ToString.php b/src/Symfony/Component/Validator/Tests/Fixtures/ToString.php
index 714fdb9e98f5f..2512066bafc87 100644
--- a/src/Symfony/Component/Validator/Tests/Fixtures/ToString.php
+++ b/src/Symfony/Component/Validator/Tests/Fixtures/ToString.php
@@ -15,7 +15,7 @@ class ToString
 {
     public $data;
 
-    public function __toString()
+    public function __toString(): string
     {
         return 'toString';
     }
diff --git a/src/Symfony/Component/VarDumper/Cloner/Data.php b/src/Symfony/Component/VarDumper/Cloner/Data.php
index 55baec5dde603..fd7a18a752385 100644
--- a/src/Symfony/Component/VarDumper/Cloner/Data.php
+++ b/src/Symfony/Component/VarDumper/Cloner/Data.php
@@ -106,11 +106,17 @@ public function getValue($recursive = false)
         return $children;
     }
 
+    /**
+     * @return int
+     */
     public function count()
     {
         return \count($this->getValue());
     }
 
+    /**
+     * @return \Traversable
+     */
     public function getIterator()
     {
         if (!\is_array($value = $this->getValue())) {
@@ -136,6 +142,9 @@ public function __isset($key)
         return null !== $this->seek($key);
     }
 
+    /**
+     * @return bool
+     */
     public function offsetExists($key)
     {
         return $this->__isset($key);
diff --git a/src/Symfony/Component/VarDumper/Cloner/Stub.php b/src/Symfony/Component/VarDumper/Cloner/Stub.php
index 27dd3ef32c4df..7e9eb6d59f519 100644
--- a/src/Symfony/Component/VarDumper/Cloner/Stub.php
+++ b/src/Symfony/Component/VarDumper/Cloner/Stub.php
@@ -44,7 +44,7 @@ class Stub
     /**
      * @internal
      */
-    public function __sleep()
+    public function __sleep(): array
     {
         $properties = [];
 
diff --git a/src/Symfony/Component/VarExporter/Tests/VarExporterTest.php b/src/Symfony/Component/VarExporter/Tests/VarExporterTest.php
index d80c2858ee91e..d62639095e2a6 100644
--- a/src/Symfony/Component/VarExporter/Tests/VarExporterTest.php
+++ b/src/Symfony/Component/VarExporter/Tests/VarExporterTest.php
@@ -209,7 +209,7 @@ public function provideExport()
 
 class MySerializable implements \Serializable
 {
-    public function serialize()
+    public function serialize(): string
     {
         return '123';
     }
@@ -227,7 +227,7 @@ class MyWakeup
     public $baz;
     public $def = 234;
 
-    public function __sleep()
+    public function __sleep(): array
     {
         return ['sub', 'baz'];
     }
@@ -305,7 +305,7 @@ public function setFlags($flags)
 
 class GoodNight
 {
-    public function __sleep()
+    public function __sleep(): array
     {
         $this->good = 'night';
 
@@ -395,7 +395,7 @@ public function unserialize($str)
 
 class Php74Serializable implements \Serializable
 {
-    public function __serialize()
+    public function __serialize(): array
     {
         return [$this->foo = new \stdClass()];
     }
@@ -405,7 +405,7 @@ public function __unserialize(array $data)
         list($this->foo) = $data;
     }
 
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException();
     }
@@ -415,7 +415,7 @@ public function __wakeup()
         throw new \BadMethodCallException();
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         throw new \BadMethodCallException();
     }
diff --git a/src/Symfony/Component/Workflow/TransitionBlockerList.php b/src/Symfony/Component/Workflow/TransitionBlockerList.php
index 8569b28fea36c..f7f4a63ea303a 100644
--- a/src/Symfony/Component/Workflow/TransitionBlockerList.php
+++ b/src/Symfony/Component/Workflow/TransitionBlockerList.php
@@ -63,7 +63,7 @@ public function isEmpty(): bool
      *
      * @return \ArrayIterator|TransitionBlocker[]
      */
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->blockers);
     }