How to implement COMET with PHP

Comet is a programming technique that enables web servers to send data to the client without having any need for the client to request it. This technique will produce more responsive applications than classic AJAX. In classic AJAX applications, web browser (client) cannot be notified in real time that the server data model has changed. The user must create a request (for example by clicking on a link) or a periodic AJAX request must happen in order to get new data fro the server.

I will now explain how to implement Comet with PHP programming language. I will demonstrate it on two demos which uses two techniques: the first one is based on hidden <iframe> and the second one is based on classic AJAX non-returning request. The first demo will simply show the server date in realtime on the clients and the second demo will display a mini-chat.

Comet with iframe technique : server timestamp demo

We need:

A PHP script that will handle the persistent http request (backend.php)
A HTML file that will load Javascript code and that will show the data coming from the server (index.html)
The prototype library that will help us to write simple JS code

To understand how it works this schema should help:
How COMET works

The backend script (PHP)

This script will do an infinite loop and will return the server time as long as the client is connected. Call it backend.php:

<?php

header("Cache-Control: no-cache, must-revalidate");
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
flush();

?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>Comet php backend</title>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>

<script type="text/javascript">
  // KHTML browser don't share javascripts between iframes
  var is_khtml = navigator.appName.match("Konqueror") || navigator.appVersion.match("KHTML");
  if (is_khtml)
  {
    var prototypejs = document.createElement('script');
    prototypejs.setAttribute('type','text/javascript');
    prototypejs.setAttribute('src','prototype.js');
    var head = document.getElementsByTagName('head');
    head[0].appendChild(prototypejs);
  }
  // load the comet object
  var comet = window.parent.comet;
</script>

<?php

while(1) {
  echo '<script type="text/javascript">';
  echo 'comet.printServerTime('.time().');';
  echo '</script>';
  flush(); // used to send the echoed data to the client
  sleep(1); // a little break to unload the server CPU
}

?>


</body>
</html>

The client script (HTML)

This HTML document first load the prototype library in the <head> tag, then it create the tag that will contains the server timer <div id="content"></div>, and finally it create a comet javascript object that will connect the backend script to the time container tag.

The comet object will create some invisible iframe tags (depends on the web browser). These iframes are in charge to create the background persistent http connection with the backend script. Notice: this script do not handle possible connection problems between client and server.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Comet demo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <script type="text/javascript" src="prototype.js"></script>
  </head>
  <body>
    <div id="content">The server time will be shown here</div>

<script type="text/javascript">
var comet = {
  connection   : false,
  iframediv    : false,

  initialize: function() {
    if (navigator.appVersion.indexOf("MSIE") != -1) {

      // For IE browsers
      comet.connection = new ActiveXObject("htmlfile");
      comet.connection.open();
      comet.connection.write("<html>");
      comet.connection.write("<script>document.domain = '"+document.domain+"'");
      comet.connection.write("</html>");
      comet.connection.close();
      comet.iframediv = comet.connection.createElement("div");
      comet.connection.appendChild(comet.iframediv);
      comet.connection.parentWindow.comet = comet;
      comet.iframediv.innerHTML = "<iframe id='comet_iframe' src='./backend.php'></iframe>";

    } else if (navigator.appVersion.indexOf("KHTML") != -1) {

      // for KHTML browsers
      comet.connection = document.createElement('iframe');
      comet.connection.setAttribute('id',     'comet_iframe');
      comet.connection.setAttribute('src',    './backend.php');
      with (comet.connection.style) {
        position   = "absolute";
        left       = top   = "-100px";
        height     = width = "1px";
        visibility = "hidden";
      }
      document.body.appendChild(comet.connection);

    } else {
    
      // For other browser (Firefox...)
      comet.connection = document.createElement('iframe');
      comet.connection.setAttribute('id',     'comet_iframe');
      with (comet.connection.style) {
        left       = top   = "-100px";
        height     = width = "1px";
        visibility = "hidden";
        display    = 'none';
      }
      comet.iframediv = document.createElement('iframe');
      comet.iframediv.setAttribute('src', './backend.php');
      comet.connection.appendChild(comet.iframediv);
      document.body.appendChild(comet.connection);

    }
  },

  // this function will be called from backend.php  
  printServerTime: function (time) {
    $('content').innerHTML = time;
  },

  onUnload: function() {
    if (comet.connection) {
      comet.connection = false; // release the iframe to prevent problems with IE when reloading the page
    }
  }
}
Event.observe(window, "load",   comet.initialize);
Event.observe(window, "unload", comet.onUnload);
</script>

</body>
</html>

Download it

Here is the tar.gz archive of this demo.

Online demo

Here is the online demo.

Comet with classic AJAX : litte chat demo

As on the above technique, we need:

A file to exchange data (data.txt)
A PHP script that will handle the persistent http request (backend.php)
A HTML file that will load Javascript code and that will show the data coming from the server (index.html)
The prototype library that will help us to write simple JS code

The backend script (PHP)

This script will do two things:

Write into data.txt when new messages are sent
Do an infinite loop as long as data.txt file is unchanged

<?php

$filename  = dirname(__FILE__).'/data.txt';

// store new message in the file
$msg = isset($_GET['msg']) ? $_GET['msg'] : '';
if ($msg != '')
{
  file_put_contents($filename,$msg);
  die();
}

// infinite loop until the data file is not modified
$lastmodif    = isset($_GET['timestamp']) ? $_GET['timestamp'] : 0;
$currentmodif = filemtime($filename);
while ($currentmodif <= $lastmodif) // check if the data file has been modified
{
  usleep(10000); // sleep 10ms to unload the CPU
  clearstatcache();
  $currentmodif = filemtime($filename);
}

// return a json array
$response = array();
$response['msg']       = file_get_contents($filename);
$response['timestamp'] = $currentmodif;
echo json_encode($response);
flush();

?>

The client script (HTML)

This HTML document first load the prototype library in the <head> tag, then it create the tag <div id="content"></div> that will contains the chat messages comming from data.txt file, and finally it create a comet javascript object that will call the backend script in order to watch for new chat messages.

The comet object will send AJAX requests each time a new message has been received and each time a new message is posted. The persistent connection is only used to watch for new messages. A timestamp url parameter is used to identify the last requested message, so that the server will return only when the data.txt timestamp is newer that the client timestamp.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Comet demo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <script type="text/javascript" src="prototype.js"></script>
  </head>
  <body>

<div id="content">
</div>

<p>
  <form action="" method="get" onsubmit="comet.doRequest($('word').value);$('word').value='';return false;">
    <input type="text" name="word" id="word" value="" />
    <input type="submit" name="submit" value="Send" />
  </form>
</p>

<script type="text/javascript">
var Comet = Class.create();
Comet.prototype = {

  timestamp: 0,
  url: './backend.php',
  noerror: true,

  initialize: function() { },

  connect: function()
  {
    this.ajax = new Ajax.Request(this.url, {
      method: 'get',
      parameters: { 'timestamp' : this.timestamp },
      onSuccess: function(transport) {
        // handle the server response
        var response = transport.responseText.evalJSON();
        this.comet.timestamp = response['timestamp'];
        this.comet.handleResponse(response);
        this.comet.noerror = true;
      },
      onComplete: function(transport) {
        // send a new ajax request when this request is finished
        if (!this.comet.noerror)
          // if a connection problem occurs, try to reconnect each 5 seconds
          setTimeout(function(){ comet.connect() }, 5000); 
        else
          this.comet.connect();
        this.comet.noerror = false;
      }
    });
    this.ajax.comet = this;
  },

  disconnect: function()
  {
  },

  handleResponse: function(response)
  {
    $('content').innerHTML += '<div>' + response['msg'] + '</div>';
  },

  doRequest: function(request)
  {
    new Ajax.Request(this.url, {
      method: 'get',
      parameters: { 'msg' : request 
    });
  }
}
var comet = new Comet();
comet.connect();
</script>

</body>
</html>

Download it

Here is the tar.gz archive of this demo.

Online demo

Here is the online demo.