วิธีวนลูปพารามิเตอร์ ของ Ajax ทำยังไงอ่ะครับ รบกวนทุกท่านด้วยครับ

function xxxx() {
		  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; }
		 	//alert(document.getElementById("cpd_type").value);
		  var url = 'xxxxxxx.php';
		 
   		  var pmeters = "tnum=" + encodeURI(document.getElementById("num").value)   ;     ///;วิธีวนลูปพารามิเตอร์
			alert(pmeters);
			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("div1").innerHTML = "<center><img src=img/ajax-loader.gif /></center>";
				  }

				 if(HttPRequest.readyState == 4) // Return Request
				  {
					setTimeout(function(){
				   document.getElementById("div1").innerHTML = HttPRequest.responseText;
				 }, 500);
				  }
				
			}
	   }