如何从Wikihow中使用此类销毁会话？

I'm following this tutorial: http://www.wikihow.com/Create-a-Secure-Session-Managment-System-in-PHP-and-MySQL

And I want to know how can I destroy the session...I've tried:

$session = new session();
$session->start_session('_s', false);
$session->destroy(session_id());

But no luck, it doesn't display any errors and the session is still opened.