In a new post to the ThinkPHP blog today there's a look at combining Comet with PHP to make a simple method for the client to talk back to the server outside the usual methods.

If a couple of users have opened the application there are already some hundred or thousand requests per second. The outcome of this is a big load for your server and a highly increased traffic - your server will in a senseless way be overloaded. In conclusion, our problem is the enormous amount of polling without knowing whether the server really wants to send a new push. Let's turn the initial situation around. And we get the solution to our problem: Comet.

With Coment, the model changes and the request is "cached" on the server-side automatically in a single Comet instance. Coordinate this with another recommendation of theirs, a PHP socket server, and you can do some pretty interesting things.