Well, I have doubts about this technology, more precisely in its implementation, can not find good examples on the Internet, as it involves javascript and php, only, you would have some links where I can find this stuff really works and that ?
A great source for all things Comet is Comet Daily. Unfortunately it's not updated all that often any more but there are some fantastic old articles in there. It's contributed to by guys that have been developing Comet solutions for over 10 years.
Comet seems to get incorrectly bundled as meaning just one particular connection mechanism is used, but it's actually a paradigm for realtime push from server to client. Comet servers can use HTTP Streaming, HTTP Long-Polling, classic polling and WebSockets.
If you are interested in the latest Push Technology then you should take a look at WebSockets which is a standardised approach to not only server to client push, but also bi-directional realtime communication between servers and clients (web browsers and other clients).
Some current trending push technologies are:
If you want to use PHP you could struggle to build an application using realtime Push which will scale above a small number of connections. Have a read through this question on concurrency - How to implement event listening in PHP for more information.