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
本函数还未编写文档，仅有参数列表。
此函数没有参数。
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.