Turn based web game in PHP - communication problem

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PHP

Ajax

MySQL

Hi all!

I am working on a turn based web game in PHP5.

This is pretty simple game, a kind of board game: two people join a "session" and they play until one of them wins.

My problem in nutshell:

* User A and User B play a game.

* User A finished his turn
* Request will be sent to the server to perform necessary operations.
* Now it is time for User B to move..

But how could I notify User B about this?

I mean, now the server has to communicate with the other user, the one that is inactive, not the one that initiated the request.

I know that this could be implemented using some kind of periodic AJAX call that checks whether or not the opponent finished his turn etc, but such a thing generates a huge number of requests.

Isn't there a better way to solve this?

I'm thinking of something like this:

* User A's turn ends
* Server saves his score
* Server contacts User B
* User B's turn gets started.

Is this possible using PHP? Should I use comet style AJAX? Or is there a better way to do this?

Any help would be appreciated!

Thanks in advance!