You'll need to be familiar with the AJAX techniques, and you'll probably need to use server-push. This is not straightforward as browsers (other than Mozilla) don't have any support for XMLHttpRequest server-push. Therefore, you need the "long-lived-iframe-requests" hack or something.
It's not easy on the server-side easy, because PHP just isn't designed for this sort of thing. A prototype I wrote used unix datagram sockets for communication between multiple PHP instances; this is tricky to get right as it's concurrent programming.
The makers of this sort of thing may just throw in the towel and write a custom single-threaded web server instead, as it would make it very much easier.
Mark
