in reply to How do i know if an user is not on my site anymore??

Just for the sake of the argument I am going to assume that you want to implement something like the "Other Users" nodelet. There is really no good way to do this. You could make every user log in/out, but users are irresponsible, and would fail to logout.

The usual method is to log what time the user last accessed a page, and then when you need to generate a list of logged in users, you go through the log diplay only users that have logged in in the last N minutes.

If you had the times stored in a database you could do:

SELECT username FROM users WHERE (last_access > $time)
$time would be the current time minus the "inactivity threshold".