Short: Is there a way to push data from the server to a logged in user session without that user needing to make an HTTP request?
Long(er):<
ScaleDrone lets you push from PHP to JavaScript using a PHP API client.
In modern browsers, Websockets or in older browsers, "long polling". There are also libraries that can abstract some of this (like comet).
A very common library in use now is socket.io which automatically connects with the best technology available, using webSockets if available and falling back to other technologies like long polling if not available.
Some other references:
How do I implement basic "Long Polling"?
COMET (server push to client) on iPhone
More on Long Polling
Is there a difference between long-polling and using Comet
Pushing the web with websockets
Oh HTML5, Push Data to my Mobile
I recommend Socket.io
It leverages
WebSocket, FlashSocket, HTML file(for IE), XHR long-polling, JSONP-polling
It is with Node.js which means it is on non-blocking nature.
In realtime feature, concurrency is a big issue
so the non-blocking nature of Node.js and Socket.io would help you