ส่ง ค่าจาก php ไปที่ ajax ไม่เห้นค่าค่ะ....ถามผู้รู้หน่อยค่ะ จะเขียน code ส่งตัว ค่า Y ผ่านไปยัง ตัว ajax ทุกอย่าง work หมดยกเว้น

<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 = 'chk_login.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 = "<img src='img/loading-medium.gif' width='24' height='24' />";
				}

				if(HttPRequest.readyState == 4) // Return Request
				{
				alert(HttPRequest.responseText);
					if(HttPRequest.responseText == "chgpwd")
					{
						window.location = 'main.php';
					}
					else if(HttPRequest.responseText == "main")
					{
						window.location = 'main.php';
					}
					else
					{
					document.getElementById("mySpan").innerHTML = HttPRequest.responseText;
					}
				}
				
			}

	   }
	</script>

                                                               document.getElementById("mySpan").innerHTML = HttPRequest.responseText;
				
				
				if(document.getElementById("mySpan").innerHTML == 'Y')
				{
				window.location.href  = 'main.php';
				}