Calling a function/method with invalid parameter, eg. parameters that does not pass parameter parsing internally all return NULL. In this case FALSE is returned if something fails after the parameter parsing have passed.

I never knew that functions/methods that fail parameter validation will always return NULL. Thanks for clearing that up for me!

But how is it that an uncallable callback is detected as a parameter failure during the internal parameter validation? I would think it would be checking for $callback to be a "string" or "array(2)" (vs. something obviously wrong, i.e., a "resource"). After that, the check for an invalid callback would be detected once call_user_func_array() attempts to call it. In which case, it would "fail" to complete its task and return FALSE.

If the "callback" type is validated fully outside of call_user_func_array()'s control--and if the change I'm suggesting is impossible to make, i.e., if other functions besides call_user_func*() will crash if given an invalid callback--could the documentation for call_user_func* be updated to read:

"Returns the function result, NULL for an invalid callback, and FALSE for all other errors."?

That addition would have saved me a lot of development time. My code was expecting FALSE from call_user_func_array() OR the callback to mean "stop the callback loop" for any reason. (NULL from the callback function is acceptable.) I have fixed my code by calling the is_callable() function first.

Thank for your help!