Actualizar PHP+AJAX

[Requenaeo]

hola ahora neceto su ayuda aqui esta un codigo que revise y probe y reacciona perfctamente pero solo fncia con un archov de texteo y quiero que reaccione con uno de php aqui estanm los codigo:

Código Javascript:

<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>
 

aqui esta el php llamada backend.php

Código PHP:

<?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();
 
?>
 

ahora no se y e intentado pero no da resultado por favor ayudenme!!