Stay logged in, or not !

[eluser]rogierb[/eluser]
The first question that pops in my mind is: How do you know someone has left?
That, to me, seems like the million dollar question.

Most session systems are based on time. When someone hasn't been active for a specific period of time, the session is destroyed. When a user logges off, their session is destroyed. This means some form of user interaction to signal the user is leaving.

You want to destroy the session when someone leaves. If you answer my first question, you will have found your answer.