Description:
------------
It would be useful to be able to use the is_callable() function on an object that overloads with __call(). Perhaps something like a magic __callable() or __is_callable() method that would return true or false on whether or not the method is callable. 

Reproduce code:
---------------
<?php
	class MyClass
	{		
		public function __callable( $sMethod )
		{
			return( is_callable( array( $this, $sMethod ) ) );
		}
		
		private function MyFunction() {}
		
	}
	
	
	$oMyClass = new MyClass();

	if( is_callable( $oMyClass, "MyFunction" ) )
	{
		echo "Callable!";
	}
?>

Expected result:
----------------
Callable!

Actual result:
--------------
[no output]