Setting a timed-session

Hi all. This may be a bit elementary, however, I am confused on the functionality of it. I am wanting to create a session that logs out after a certain amount of time, no matter if the user is doing something or not. I have used the function set_time_limit(), but I don't know if I'm using it properly. What I do is whenever a certain page is accessed, I call that function. I tested it with 10 seconds, but nothing seemed to happen. Is it supposed to kill the session and all of the $_SESSION variables? Or what exactly is it supposed to do? I looked on the php site, but I'm still a bit baffled? Any insight? http://us4.php.net/function.set-time-limit Thanks in advance, ~Philip