[eluser]kyko[/eluser]
Thanks for the suggestions guys,
What i did was to implement a DB Sessions. In that way, i can track all users that are logged in (that is if session data is not NULL) and users that are guests (that is if session data is null). I have also implemented the tip from Sumon, that in every page refresh, i have to check whether the last activity exceeds the allowable idle time. All sessions that exceeds the predefined time will be deleted. If, however, the user is logged in, the last_activity should be updated during page refresh.
Again, thanks for all your suggestions.