public function unregister_hook($hook, $callback)
    {
        $callback_id = array_search($callback, (array) $this->handlers[$hook]);
        if ($callback_id !== false) {
            // array_splice() removes the element and re-indexes keys
            // that is required by the 'for' loop in exec_hook() below
            array_splice($this->handlers[$hook], $callback_id, 1);
        }
    }