Updating DB when a user leaves my site...

Hi,
I need to keep track of users connected to my site at one time. So when a user logs in its easy to just update a counter of users. But, when a user leaves my site, they might just close the page, and not log out, so...how do I update the db if they just close the page?

Is there a server side function that can be called onExit of a web site???

Thanks in advance
:)

You could either do it when the session times out (by default, it's about 20 minutes).

Or, using javascript, when the user closes the current window, a tiny window opens up which simply updates the database that the user has left the "building"

HTH

whats the Javascrip function for when a window has been closed?

Also, what about if the user exits my site by entering a new url, and doesnt close the window?