Apparently this does not return true for callback arguments to many of the internal functions, such as array_map and array_walk.
(PHP 5 >= 5.4.0, PHP 7)
ReflectionParameter::isCallable — Returns whether parameter MUST be callable
This function is currently not documented; only its argument list is available.
This function has no parameters.
Returns TRUE if the parameter is callable, FALSE if it is
not or NULL on failure.
Apparently this does not return true for callback arguments to many of the internal functions, such as array_map and array_walk.