Whats the easiest way to determine if a user is online? (PHP/MYSQL)

Question

Is there a way I can piggy back sessions to know if the user is online?

I.e: use logs on, I set a $_SESSION variable, user times out- cookie Garbage collector update

Answer 1

Store the time-zone in the table, and use it in a calculation to find that users local time in comparison to the local time of the user viewing the page.

Edit:

Better yet, store all times as the server time, and base all calculations relative only to the server time.