sessions and trans-sid problem/question

I've made a site in PHP and on some pages a user needs to log in first before gaining access to the page. (i.e. there is a log in page). Once the user has logged in I keep that fact in a session variable so that he doesn't need to log in again. However I have found out that if: 1- the user logs in 2- bookmarks the page 3- closes the browser 4- opens the browser 5- goes to the saved bookmark page He has access to the page. I.e. the session did not close/terminate when he closed his browser ... In Netscape 7 I have checked the stored cookie and it is set to expire at the end of the session (which is the default I think), so I don't understand why the PHP thinks the session is still opened ... Help! Jc