Auto logout?

I have a login, and when users are logged on, it should show in some way on the page, that's not the problem, it is to show that they are logged off when they are, often people forget to press the logout button, and the mysql database is unchanged, saying they are still logged on, how can I do so it logs out when a user leaves the page, if the page is closed or something like that, need suggestions, sorry for my currently bad english, but I'll blame the heat ;) Håkan