if（xmlhttp.readyState == 4 && xmlhttp.status == 200）这个条件不起作用

Actually I am working on a chatbox....But It doesn't meet this condition. Here is the code

function submitChat() {
  if (form1.users.value == "" || form1.messege.value == "") {
    alert("Please fill the all feild");
    return;
  }
  var users = form1.users.value;
  var messege = form1.messege.value;
  var xmlhttp = new XMLHttpRequest();
  xmlhttp.onreadystatechange = function() {
    //Here is the problem may be
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
      document.getElementByID('shouts').innerHTML = xmlhttp.responseText;
    }
  }
  xmlhttp.open('GET', 'process.php?users' + users + '&messege' + messege, 'true');
  xmlhttp.send();

Here these variable shows undefined..

$users = $_REQUEST['users'];
$messege =  $_REQUEST['messege'];

You've left out the = between the parameter names and their values.

Also, when you're sending a URL parameter that could include special characters, you need to encode it properly, using encodeURIComponent()

xmlhttp.open('GET', 'process.php?users=' + encodeURIComponent(users) + '&messege=' + encodeURIComponent(messege), true);

Note also that the third argument should be a boolean, not a string (although since any non-empty string is truthy, 'true' will have the same effect as true, but 'false' would not be equivalent to false).

It's also not usually a good idea to use a URL parameter for long inputs like messages, you should use POST data instead.

xmlhttp.open('POST', 'process.php', true);
xmlhttp.send('users=' + encodeURIComponent(users) + '&messege=' + encodeURIComponent(messege));