Need help to create a smart chat application in PHP?

[Mahen]

I just want to know that, is there a way by which we can create a chat application in a structure where server it self ping client machine & send him it's message received.

I have checked various chat applications and all are working in the same way where a request from client machine is send to server to check messages received & if a message is exists there then it collects that message & shows on chat window (in some application this is done by using AJAX).

I want a system where all clients need not to request to the server to check their contents, because this will create extra unnecessary overhead on server machine.

Any Idea ????????????
I have checked many open source chat scripts but they all don't supports my idea as i said:
"server it self ping client machine & send him it's message received" it will only happened when a new message is received by server for a client (ofcource when client is online).

[Pratap]

In PHP their will be Free Open Source Chat software are their gooogle it