EventSource: The Glorified Long Polling Machine

event-source.html

Open your console.
<script src="event-source.js"></script>

event-source.js

document.addEventListener("DOMContentLoaded", function() {

  /*

  EventSource is nothing more then a glorified
  long polling machine. It will create HTTP requests 
  to a provided url of the same origin 
  (which in turn creates an `open` event ) until
  it sees a valid "text/event-stream" response body
  at which point a `message` event will be fired.

  This process will repeat until the EventSource is
  terminated my calling its close() method. 

  no "data: " message from the server should result in long polling
  `open` events being fired, followed by zero `message` events

  */

  var // declare localized references
  eventSrc  = new EventSource( "event-source.php" ), 
  handler = function( event ) {

    console.log( [ event.type, new Date(), event, event.data ] );

  },
  getReadyState = function( src ) {

    if ( src.readyState ) {
      // readyState is almost always 0, we're only interested in 
      // seeing readyState OPEN (1) ( or CLOSED (2) )
      console.log( [ src.readyState, new Date() ] );
    }

    setTimeout(function() {
      getReadyState( src );
    }, 1);
  };

  console.log( eventSrc );

  // Setup event handlers
  [ "open", "message" ].forEach( function( name ) {

    eventSrc.addEventListener( name, handler, false );

  });

  // Begin sampling the ready state
  getReadyState( eventSrc );

}, false);

event-source.php

<?php
// Make sure we are starting an event stream
header("Content-Type: text/event-stream\n\n");

// Continue in an infinite loop
while (true)
{
    // Every second we will push "hello world"
    sleep(1);
    echo "data: hello world \n\n";

    // This will ensure our data is sent provided you set this
    // in your php.ini:
    // output_buffering = off
    ob_flush();
    flush();
}

The reason for output_buffering = off, is because by default it is set to 4096 which means PHP will wait until 4096 bytes of information are stored in the buffer before it flushes, even if in your script you try to do it before this limit is reached. Alternately you could just get this script to output 4096+ bytes and you wouldn't have to change a thing in your php.ini.