checking session ID's

Hi list, it's been a while. I want to see how many users are online. I store the users $PHPSESSID in a database field (phpsessid) when they login. When they logout, the $PHPSESSID value in phpsessid is removed. I could just see how many rows have something in the phpsessid field, but if a user doesn't click the "Logout" button, then the value stays there, it doesn't get removed. So, does anyone know how else I could do this? If I could check to see if an $PHPSESSID is valid, I could look through the database and check to see if the value in the phpsessid field is still valid. Any ideas? thanks, Tyler