Grazie 8matt5 per il suggerimento ma, molto probabilmente, "canno" qualcosa.
Questo č il codice relativo alla pagina in cui voglio inserire la chat:

<?php
// get user name
session_start();
$user=$_SESSION['user'];
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title></title>
<link href="../StiliTabelle.css" rel="stylesheet" type="text/css" />

<script language="javascript">
/*
************************************************** ***************
AJAX-Based Chat System
Author: Alejandro Gervasio
Version: 1.0
************************************************** ***************
*/
// getXMLHttpRequest object
function getXMLHttpRequestObject(){
var xmlobj;
// check for existing requests
if(xmlobj!=null&&xmlobj.readyState!=0&&xmlobj.read yState!=4){
xmlobj.abort();
}
try{
// instantiate object for Mozilla, Nestcape, etc.
xmlobj=new XMLHttpRequest();
}
catch(e){
try{
// instantiate object for Internet Explorer
xmlobj=new ActiveXObject('Microsoft.XMLHTTP');
}
catch(e){
// Ajax is not supported by the browser
xmlobj=null;
return false;
}
}
return xmlobj;
}
// check status of sender object
function senderStatusChecker(){
// check if request is completed
if(senderXMLHttpObj.readyState==4){
if(senderXMLHttpObj.status==200){
// if status == 200 display chat data
displayChatData(senderXMLHttpObj);
}
else{
alert('Errore di comunicazione: '+ senderXMLHttpObj.statusText);
}
}
}
// check status of receiver object
function receiverStatusChecker(){
// if request is completed
if(receiverXMLHttpObj.readyState==4){
if(receiverXMLHttpObj.status==200){
// if status == 200 display chat data
displayChatData(receiverXMLHttpObj);
}
else{
alert('Errore di comunicazione: '+ receiverXMLHttpObj.statusText);
}
}
}
// get messages from database each 5 seconds
function getChatData(){
receiverXMLHttpObj.open('GET','getchatdata.php',tr ue);
receiverXMLHttpObj.send(null);
receiverXMLHttpObj.onreadystatechange=receiverStat usChecker;
setTimeout('getChatData()',5*1000);
}
// display messages
function displayChatData(reqObj){
// remove previous messages
var mdiv=document.getElementById('messages');
if(!mdiv){return};
mdiv.innerHTML='';
var messages=reqObj.responseText.split('|');
// display messages
for(var i=0;i<messages.length;i++){
var p=document.createElement('p');
p.appendChild(document.createTextNode(messages[(messages.length-1)-i]));
mdiv.appendChild(p);
}
}
// send user message
function sendMessage(){
var user='<?php echo $user?>';
var message=document.getElementsByTagName('form')[0].elements[0].value;
if(message.length>100){message=message.substring(0 ,100)};
// open socket connection
senderXMLHttpObj.open('POST','sendchatdata.php',tr ue);
// set form http header
senderXMLHttpObj.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
senderXMLHttpObj.send('user='+user+'&message='+mes sage);
senderXMLHttpObj.onreadystatechange=senderStatusCh ecker;
}
// create messages board
function createMessageBoard(){
var mdiv=document.createElement('div');
mdiv.setAttribute('id','messages');
document.getElementsById('tdChat');
}
// create message input box
function createMessageBox(){
// create message box container
var mdiv=document.createElement('div');
mdiv.setAttribute('id','messagebox');
// create message form
var mform=document.createElement('form');
// create message box
var mbox=document.createElement('input');
mbox.setAttribute('type','text');
mbox.setAttribute('name','message');
mbox.className='msgfield';
// create 'send' button
var mbutton=document.createElement('input');
mbutton.setAttribute('type','button');
mbutton.setAttribute('value','Invia messaggio');
mbutton.onclick=sendMessage;
// create login text
var sp=document.createElement('span');
sp.appendChild(document.createTextNode('Logged in as: <?php echo $user?>'));
// append elements
mform.appendChild(mbox);
mform.appendChild(mbutton);
mform.appendChild(sp);
mdiv.appendChild(mform);
document.getElementsById('tdChat').appendChild(mdi v);
mbox.focus();
mbox.onfocus=function(){this.value='';}
}
// initialize chat
function intitializeChat(){
if(document.getElementById&&document.getElementsBy TagName&&document.createElement){
createMessageBoard();
createMessageBox();
getChatData();
}
}
// instantiate sender XMLHttpRequest object
var senderXMLHttpObj=getXMLHttpRequestObject();
// instantiate receiver XMLHttpRequest object
var receiverXMLHttpObj=getXMLHttpRequestObject();
// initialize chat
window.onload=intitializeChat;
</script>
</head>

<body>
<div align="center">
<table border="0" cellspacing="0" cellpadding="0">
<tr>
<td id="tdChat"></td>
</tr>
</table>
</div>
</body>
</html>

ma nella tabella non mi compare nč i messaggi presenti nel database (che ho verificato e ci sono), nč la casella di testo per immettere il messaggio, nč il pulsante per inviare il messaggio.

Sai dirmi dove sbaglio?