I try to get the API doing it like described in the methods-doc
> Returns the result of the callable on success, or <code>true</code> when the callable doesn't return anything.

Was inspired by doctrine, https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/EntityManager.php#L218, but it seems it doesn't make sense, right.

After thinking several times how to solve this.. there is not bullet proof way to check whether `call_user_func` returned something (because php implicitly returns `null` when no return is inside a function/method). We need to fix the docs and always return what `call_user_func` returns..