phpforfun

I am making a game with PHP and ajax but I have an issue. When a player joins a queue their user id is inserted into a database field called users. But when users either logout, close the browser, just leave the game in some form of way I need their id to be removed from that field. I know there is jquery .unload function but how reliable is that really? So could anyone help me in finding a reliable way of removing the user id from the field when they leave the game? thanks