Destroy Sessions

Hello, I am encountering a problem with sessions and them lingering around. Basically, a user logs in and they can see certain pages based on their access level. There is a logout button also that runs the session_destroy() function. If the user presses logout, there are no problems. However, if the user bookmarks the internal pages and then closes the browser without clicking logout, that session still lingers. When they reopen the browser and use their bookmark, the internal pages work! If I look at the directory where the sessions are stored, I can see the session file created when the person logs in. If they click logout (hence session_destroy()) the file gets deleted. However, if the user just closes the window, the session file remains, which allows the use of the bookmark. I am passing PHPSESSID via GET also in case people don't use cookies. Why isn't the session file getting deleted when a user closes the browser? How do I do this? Rohan :-)