determining users on a forum (php)

[Kakashi266]

Hey guys. I'm working on making my own custom forum (for the experience, not as a replacement), and I have finally run into a snag I don't think I can handle. I want to determine how many people are on the forum, just like the phpbb ones do. They say they use the last five minutes as a reference, which I could technically do, but I'm not sure how they do it. The only way I could think to do it, if the person hasn't refreshed or changed pages in five minutes they would no longer be on the forum, though they technically still would. If you guys need specific code from anything, just tell me and I'll post it. But really I'm just looking for a vague reference. Normally I like to figure these things out by myself. Thanks, guys.

Nevermind, guys. I guess someone who hasn't refreshed or changed pages withing 5 minutes isn't very active at all, and they would pop right back up if they did change.