HttPRequest.readyState == 4 ไม่ทำงานไม่ทราบว่าต้องแก้ใขยังไงครับ

<script language="JavaScript">
	   var HttPRequest = false;

	   function doCallAjax() {
		  HttPRequest = false;
		  if (window.XMLHttpRequest) { // Mozilla, Safari,...
			 HttPRequest = new XMLHttpRequest();
			 if (HttPRequest.overrideMimeType) {
				HttPRequest.overrideMimeType('text/html');
			 }
		  } else if (window.ActiveXObject) { // IE
			 try {
				HttPRequest = new ActiveXObject("Msxml2.XMLHTTP");
			 } catch (e) {
				try {
				   HttPRequest = new ActiveXObject("Microsoft.XMLHTTP");
				} catch (e) {}
			 }
		  } 
		  
		  if (!HttPRequest) {
			 alert('Cannot create XMLHTTP instance');
			 return false;
		  }
	
		  var url = 'checkLogin.php';
		  var pmeters = "tUsername=" + encodeURI( document.getElementById("txtUsername").value) +
						"&tPassword=" + encodeURI( document.getElementById("txtPassword").value );

			HttPRequest.open('POST',url,true);

			HttPRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
			HttPRequest.setRequestHeader("Content-length", pmeters.length);
			HttPRequest.setRequestHeader("Connection", "close");
			HttPRequest.send(pmeters);
			
			
			HttPRequest.onreadystatechange = function()
			{

				if(HttPRequest.readyState == 3)  // Loading Request
				{
					document.getElementById("mySpan").innerHTML = "Now is Loading...";
				}

				if(HttPRequest.readyState == 4) // Return Request //ตรงนี้มันไม่ทำงานครับ
				{
					var objReqesttext = HttPRequest.responseText;
					if(objReqesttext.toString == 'YES') 
					{
						window.location = 'checkUserLogin.php';
					}
					else
					{
						document.getElementById("mySpan").innerHTML = HttPRequest.responseText;
					}
				}
			}
	   }
</script>

<?php
$strUsername = trim($_POST["tUsername"]);
$strPassword = trim($_POST["tPassword"]);
	
	//*** Check Username ***//
	if(trim($strUsername) == "")
	{
		echo "<font color='#FFFF00'>** Plase input ".'"'."Username".'".'."</font>";
		exit();
	}
	
	//*** Check Password ***//
	if(trim($strPassword) == "")
	{
		echo "<font color='#FFFF00'>** Plase input ".'"'."Password".'".'."</font>";
		exit();
	}
	//*** Check Username & Password ***//

	$strSQL ="SELECT * FROM ตาราง WHERE username = '".$strUsername."' AND password = '".$strPassword."' ";
	$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
	$objResult = mysql_fetch_array($objQuery);
	$totalrow = mysql_num_rows($objQuery);
	if($objResult){
		
		//*** Session ***//
		$_SESSION["username"] = $strUsername;
		session_write_close();
		echo "YES";
	}
	else
	{
		echo "<font color='#FFFF00'>** Username & Password is wrong.</font>";
	}
	mysql_close($conn);
?>

<?php
$strSQL = "SELECT * FROM ตาราง WHERE username = '".SESSION."'  ";
	$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
	$objResult = mysql_fetch_array($objQuery);
	$dbResult = $objResult['member_type'];
	if($dbResult == 0){
		echo "<SCRIPT LANGUAGE='JavaScript'>
				<!--
				location = 'indexadmin.php'
				//-->
			</SCRIPT>";
		}
	elseif($dbResult == 1){
		echo "<SCRIPT LANGUAGE='JavaScript'>
				<!--
				location = 'indexuser.php'
				//-->
			</SCRIPT>";
		}
?>