Use this for a clean end.
$_SESSION = array();
session_destroy;
Setting $_SESSION to an empty array first, clears all session variables without worrying which ones they are.
quote----------- http://uk2.php.net/manual/en/function.session-destroy.php
session_destroy() destroys all of the data associated with the current session. It does not unset any of the global variables associated with the session, or unset the session cookie.
end quote----------