I'm not sure about the patch: if I revert the change but keep the test, the test fails with an undefined offset error.
This error is fixed by this patch:

--- a/src/Symfony/Component/Dotenv/Dotenv.php
+++ b/src/Symfony/Component/Dotenv/Dotenv.php
@@ -203,7 +203,10 @@ final class Dotenv
                 $this->cursor += 1 + $len;
             } elseif ('"' === $this->data[$this->cursor]) {
                 $value = '';
-                ++$this->cursor;
+
+                if (++$this->cursor === $this->end) {
+                    throw $this->createFormatException('Missing quote to end the value');
+                }
 
                 while ('"' !== $this->data[$this->cursor] || ('\\' === $this->data[$this->cursor - 1] && '\\' !== $this->data[$this->cursor - 2])) {
                     $value .= $this->data[$this->cursor];

Which means we need another test case to trigger the infinite loop and fix it I think.

I'm not sure about the patch: if I revert the change but keep the test, the test fails with an undefined offset error.

@nicolas-grekas Yes, this is because of the ErrorHandler of phpunit. The access of the uninitialized string offset triggers a notice which invokes the error handler and throws an exception. To trigger the infinite loop in the unit test you have to disable phpunit's convertNoticesToExceptions setting.

Or if you want to see the infinite loop you can try the code block that I have attached to #34642.

This error is fixed by this patch:

Your patch fixes the error, but with a wrong offset in the error hint.
Try this script with your fix:

require __DIR__.'/vendor/autoload.php';

$testString = 'FOO="x"
DOO="y
VOO="z"';

$dotenv = new \Symfony\Component\Dotenv\Dotenv();

$dotenv->parse($testString);

the output will look like:

$ php test.php 
PHP Fatal error:  Uncaught Symfony\Component\Dotenv\Exception\FormatException: Missing quote to end the value in ".env" at line 2.
...O="x"\nDOO="y\nVOO="z"...
                        ^ line 2 offset 22 in /var/www/test-dotenv/vendor/symfony/symfony/src/Symfony/Component/Dotenv/Dotenv.php:426

This makes it harder to find the broken line, because the hint in the output points to the last character of the whole env file.
This is why I have introduced the $prevLf variable.

My patch will output

$ php test.php 
PHP Fatal error:  Uncaught Symfony\Component\Dotenv\Exception\FormatException: Missing quote to end the value in ".env" at line 2.
...FOO="x"\nDOO="y\nVOO="z"...
                 ^ line 2 offset 14 in /var/www/test-dotenv/vendor/symfony/symfony/src/Symfony/Component/Dotenv/Dotenv.php:426

I think this makes it easier to spot the issue.

Even better would be if the output sayd line 2 offset 6 but for this to work I'd have to use another way to throw the exception.

Your patch fixes the error, but with a wrong offset in the error hint.

The error looks correct to me. Multi-line quoted strings are possible, we cannot assume anything about their content.

Okay, how should be proceed with this PR? Should I revert my fix and only provide the unit test, or should I change my fix to the version suggested by you?

I'd be in favor of adopting my patch if you don't mind too much, and adapt tests accordingly.

ping @Naitsirch

Okay, I'll try to update my PR and will contact you in the next few days.

@nicolas-grekas I have updated the patch following your suggestion and adjusted the unit test.

Thank you @Naitsirch.