I propose keeping the above $placeholder index and use this here:

@@ -71,6 +71,12 @@ class EnvPlaceholderParameterBag extends ParameterBag
      */
     public function mergeEnvPlaceholders(self $bag)
     {
-        $this->envPlaceholders = array_merge_recursive($this->envPlaceholders, $bag->getEnvPlaceholders());
+        if ($newPlaceholders = $bag->getEnvPlaceholders()) {
+            $this->envPlaceholders += $newPlaceholders;
+
+            foreach ($newPlaceholders as $env => $placeholders) {
+                $this->envPlaceholders[$env] += $placeholders;
+            }
+        }
     }
 }

@nicolas-grekas Can you maybe explain the reason for the change here a bit more, I'm not seeing it.
With $this->envPlaceholders += $newPlaceholders; you're saying merge the new into the existing, fair enough. Then it's looping through each of the new ones and merging the existing ones with it? Won't this already have been done by the first merge?

The array is recursive, but the + isn't. So: the 1st + adds the new keys to the property, then + in the foreach adds the new entries to previously existing keys.

In that case why not just use a single array_merge then?
(edit) bad suggestion. that would not merge the individual elements from, instead just overwrite them using the second value

@nicolas-grekas I was trying to make a test for this but I need your input.

param bag A has placeholders:
'DB_USER' => ['env_DB_USER_1']

param bag B has placeholders:
'DB_USER' => ['env_DB_USER_1'], 'DB_HOST_1' => ['env_DB_HOST_1]

After the merge I would expect param bag A to have placeholders:
'DB_USER' => ['env_DB_USER_1'], 'DB_HOST_1' => ['env_DB_HOST_1]

right?

Now lets make things a little more interesting with param bag C which has a different unique identifier for one of the placeholders:

'DB_USER' => ['env_DB_USER_2']

So now we go and merge that into param bag A (after merging B). Do you expect:

A) 'DB_USER' => ['env_DB_USER_2'], 'DB_HOST_1' => ['env_DB_HOST_1]

or

B) 'DB_USER' => ['env_DB_USER_1', 'env_DB_USER_2'], 'DB_HOST_1' => ['env_DB_HOST_1]

If you're expecting B) like I am then I'll happily make your changes (which work as expected) once I reintroduce the $this->envPlaceholders[$env][$placeholder] = $placeholder; from above. If I left it without it your suggestion wouldn't work. The changes on my branch as they are now work in either case. I'm happy with both, using += looks nice and neat.

Just let me know the decision and i'll push the changes and the new test.

Sorry for the delay, just want to be sure I get this right this time.

B) is the only right answer. We though array_merge_recursive would do it, but you proved us wrong. Now we're trying to achieve B), nothing else :)

just want to be sure I get this right this time.

I ran my proposal with your tests, so we're safe, isn't it ;)

Pushed changes, and this time I actually tried it out on a project. I think I put too much faith in the tests last time. That or I didn't write enough tests...
Are there tests for actual retrieving of a parameter after the bag has been merged?

Are there tests for actual retrieving of a parameter after the bag has been merged?

I don't think so