<html>
<body>
<script type="text/javascript">
function ajaxFunction(str)
{
var httpxml;
try
  {
  // Firefox, Opera 8.0+, Safari
  httpxml=new XMLHttpRequest();
  }
catch (e)
  {
  // Internet Explorer
  try
    {
    httpxml=new ActiveXObject("Msxml2.XMLHTTP");
    }
  catch (e)
    {
    try
      {
      httpxml=new ActiveXObject("Microsoft.XMLHTTP");
      }
    catch (e)
      {
      alert("Your browser does not support AJAX!");
      return false;
      }
    }
  }
///////End of First part- Checking browser support//////
////// Start of receiving data from server //////
function stateChanged() 
    {
    if(httpxml.readyState==4)
      {
      document.myForm.time.value=httpxml.responseText;
      }
    }
///////  End of receiving data from server //////
///////  Start of posting data to server ///////
var url="ajax-get-testck.php";
url=url+"?txt="+str;
url=url+"&sid="+Math.random();
httpxml.onreadystatechange=stateChanged;
httpxml.open("GET",url,true);
httpxml.send(null);
  }
//////// End of posting data to server ////////
</script>
<form name="myForm">
Name: <input type="text"
onkeyup="ajaxFunction(this.value);" name="username" />
Time: <input type="text" name="time" />
</form>
</body>
</html>
At the server end we have the file ajax-get-testck.php, check this line 
var url="ajax-get-testck.php";
The code inside the file ajax-get-testck.php is here. 
<?Php
$in=$_GET['txt'];
if(strlen($in) < 20){$in=$in." Welcome ";}
else{$in=" Too Long Data";}
echo $in; // returned Message 
?>
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