Session Checking

I want to have an alert pop up to let the user know their session is about to expire. Would the best approach be to do a timer in Javascript or check it in PHP. I'm storing session data in a MySQL database so I can know when a session will expire. If I check it through PHP will the session expiration refresh when I check it? Thanks! Floyd BTW, is Arby's having a roast beef sale or something? Everyone on the list seems to be gone!