$HTTP_SESSION_VARS with unset()

when someone logs in, it sets $HTTP_SESSION_VARS with the appropriate information. to log them out, I tried just using unset($HTTP_SESSION_VARS['valid_user']) but it doesn't actually remove it from the session. it will only work if valid_user was set on the same script call (for example, if it logs me in and then immediately logs me out without making another call to the script). How come $HTTP_SESSION_VARS won't destroy session variables unless they're created during the same script call? Or maybe it just has to do with if $HTTP_SESSION_VARS creates any variable before it deletes any. Thanks for your help =)