Comet & PHP: How to use Comet with a PHP Chat System?
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.
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.