I have to build a simple chat system in PHP using Comet.
I don\'t know what would be the best approach to this project.
What is the best technique
php is not a good match for comet, because there is no way to have light weight request handlers. You would have to lock up a full process for each user connected. It could work with very few users, but not for anything with even moderate traffic.
You may want to look at WebChat2 This project uses comet, AJAX, and a custom HTTP server to communicate with IRC via sockets.
chart & Long point-by-point discussion
Unfortunately, there's no information for PHP in there, though you could potentially wrap your existing back-end with a comet server on some other platform.
There is a similar SO question about this, looks like FastCGI is your best bet.