if (!function_exists('array_diff_key'))
{
    function array_diff_key()
    {
        $arrs = func_get_args();
        $result = array_shift($arrs);
        foreach ($arrs as $array) {
            foreach ($result as $key => $v) {
                if (array_key_exists($key, $array)) {
                    unset($result[$key]);
                }
            }
        }
        return $result;
   }
}

Fallback Function for versions earlier then php5

array_diff_key() for PHP4

php, array

PHP