Session cookies

Hi, When the user logs in , i create a session with session varialbles, the session cookie is saved on clients computer. When i log off i say session_unset(); session_destroy(); setcookie(session_name()); The session in the tmp is deleted , but the cookie is still there , i know this because when i login , the same session id is used ! Why is that ? The session id changes when you close the browser , as the default is 0. Thanks Shaun