Description:
------------
in_array() returns true if needle exists as key with int(0) as value, but should return false because needle doesn't exist as value.

Reproduce code:
---------------
<?php
var_dump( in_array('a', array('a' => 1) ) );
var_dump( in_array('a', array('a' => 0) ) );
?>

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


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