Session Time to Live

Hi all, Is there any way of setting a time to live on sessions stored in php? I have tried session_cache_expire(10); but the SIDs are still valid and return session variables after that time. I am trying to avoid people 'hacking' my user's sessions. A long winded way i know would be to have a mysql table with sessions and datestamps with a crontab to clear old sessions from validity, but this means more server load and more time. Any help is appreciated, thanks. Bix