Push notifications from server to user with PHP/JavaScript

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

ScaleDrone lets you push from PHP to JavaScript using a PHP API client.