PHPStan doesn't support recursive types as they're very hard to do, but here's an idea - you could have the actual config structure here - specific keys, specific structures in those keys. It's a question of a dynamic return type extension (https://phpstan.org/developing-extensions/dynamic-return-type-extensions).

Also look into array shapes: https://phpstan.org/writing-php-code/phpdoc-types#array-shapes

Thanks in advance.
Array shapes doesn't help because keys are unknown for the Config object itself. first level is mostly a string key but values could be nested and extended nested and so on. But when running to the last element of the array and what to get it (the value), its probably not an array anymore.

i will have a look at the extensions. Thank you.

On the other hand:
Fixing the code would work except the first error. that one i cannot fix with default behavior (Line: 26)
Line: 27 add: /** @var array<int|string, mixed> $config Correct here because isset($config[$key]) succeed */

• 1 Error open.
  And this makes me think phpstan does not recocnise the type change of the value. Because incoming parameter is array|mixed. and after a successful test it could work succfully. And because: The function returns also one or the other array|mixed.
  Well, i can imagin this is much toubble for the parser :-)