Description:
------------
is_a() don't recognize class name.

Reproduce code:
---------------
<?php

namespace A;

class foobar {
    public function test() {
    }
}

$x = new foobar();

var_dump(is_a($x, 'foobar'));
var_dump(is_a($x, 'A::foobar'));
var_dump(is_a($x, '::foobar'));

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

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