ajax xmlHttpRequest get post

<script type="text/javascript">
var xmlhttp;
function loadXMLDoc(url)
{
xmlhttp=null;
if (window.XMLHttpRequest)
  {// code for all new browsers
  xmlhttp=new XMLHttpRequest();
  }
else if (window.ActiveXObject)
  {// code for IE5 and IE6
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
if (xmlhttp!=null)
  {
  xmlhttp.onreadystatechange=state_Change;
  xmlhttp.open("GET",url,true);//true     。
  xmlhttp.send(null);//null             
  //    null      POST   
  // xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded;charset=utf-8;");
  // xmlhttp.send("name="+userName+"&userId="+userId);
  }
else
  {
  alert("Your browser does not support XMLHTTP.");
  }
}

function state_Change()
{
if (xmlhttp.readyState==4)
  {// 4 = "loaded"
  if (xmlhttp.status==200)
    {// 200 = OK
    // ...our code here...
	Dispaly(); //      ，     ，    
	//  document .getElementById ("myTime").innerHTML =xmlhttp.responseText;
    }
  else
    {
    alert("Problem retrieving XML data");
    }
  }else //         ，    
  {
		document .getElementById ("myTime").innerHTML ="     ";
  }
}
 function Dispaly() //          ，      
{
	document .getElementById ("myTime").innerHTML =xmlhttp.responseText;
}
</script>