Question on session_destroy duties

Hi ppl.. i have just finished a session based site.. everything works fine.. but i have a little problem.. I want to do a session_destroy() when the guy using my site decides to leave it.. that is it, when the visitor closes its browser.. so when he/she closes its browser so i do a session_destroy and clear one table fileds i have in my mysql system... my question is.. how do i get the closing browser event. i´ve catched it using javascript but i can not call those php functions from my javascript.. please any idea of how to call some php functions when the visitor closes the browser... thkns a lot in advance. cheers..