Forming a line??

Ok, have you ever been to a place like File Planet where they have a line set up? Well I am trying to do that, I just can;t think of a way to test and see if a user is still connected to me. I considered a refreshing popup, that if the server doesn't get an updated time, then it boots the person out of line, but alot of people don't like popups. I also considered having a connection made to the person, but if they had a firewall, it would probably refuse the connection. I thought about using somethiung like passthru() and running a ping on the person, but that would work until they siged off. Does anyone have any other thoughts or is the popup the way to go?

I think the easiest way to achieve this effect is with a popup, hence why most sites who use this features go with popups. However, if you really do not want a popup, i imagine this could be done using an iframe