Description:
------------
At the moment instanceof requires the first 'parameter' to be an object of the class which is to be compared.
It would be useful to be able to check without creating the object.

Reproduce code:
---------------
<?php
class MyClass {}

$class = 'MyClass';

var_dump($class instanceof MyClass);
var_dump($$class instanceof MyClass);
var_dump(MyClass instanceof MyClass);
?>

Expected result:
----------------
bool(false)
bool(true)
bool(true)

Actual result:
--------------
bool(false)
bool(false)
bool(false)