User Authentication / only 1 login per user at any time

Hi, With most of my scripts, when I am about to write something, I have it in my head on how to do it exactly.. But with only 1 login per user at anytime, I havent found any good ways to do it, All I can think of is to set a mysql field called ³logged_in² to ³1² or ³0² if logged in or not, but the problem with that is if you close the browser , it will stay ³logged_in=1² , Is the way around that problem to do some sort of timeout code that checks all ³logged_in² in the whole table and also a field where it logged the last time the user accesses a page, to see if the user has accessed a page within 20 minutes .. Or is there a better way to do it? Thanks, Lewi