index.html<script type="text/javascript">
<!--
var time =0;
var http_request=null;    
     function HttpRequest(){    
        if (window.ActiveXObject){
            http_request = new ActiveXObject("Microsoft.XMLHTTP"); 
         }else if (window.XMLHttpRequest){
            http_request = new XMLHttpRequest();
           }
           return http_request;
     }
HttpRequest();
function autoSend(timestamp){
    alert(timestamp);
    var url = 'back.php';
    http_request.open('POST', url);
    http_request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8"); 
    http_request.send('timestamp='+ timestamp +'&num='+Math.random());
    http_request.onreadystatechange = CallBack;
}
function send(){
    var msg = document.getElementById('msg').value;
    document.getElementById('msg').value = '';
    var url = 'back.php';
    http_request.open('POST', url); 
    http_request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8"); 
    http_request.send('msg='+msg+'&num='+Math.random());
}
function CallBack(){
    if(http_request.readyState == 4) {      
            if (http_request.status == 200) { 
               var data = http_request.responseText;
               alert(data);
               var json = eval('('+ data +')');
               document.getElementById('content').innerHTML += '<div>'+ json.msg +'</div>'; 
               autoSend(json.timestamp);
            }
     }
}
window.onload = function(){
    autoSend(0);
}
//-->
</script>
<div id="content"></div>
<input type="text" id="msg"><input type="button" value="Send" onclick="send()">
back.php
<?phpset_time_limit(0);
ob_end_clean();$file = dirname(__FILE__). '/data.txt';if(!empty($_REQUEST['msg'])){
    file_put_contents($file, $_REQUEST['msg']);
    die('Write');
}$lastmodif    = isset($_REQUEST['timestamp']) ? $_REQUEST['timestamp'] : 0;
$currentmodif = filemtime($file);
while($currentmodif <= $lastmodif ){
  sleep(2);
  clearstatcache();
  $currentmodif = filemtime($file);
}$response = array();
$response['msg']       = file_get_contents($file);
$response['timestamp'] = $currentmodif;
echo json_encode($response);
flush();
?>