สอบถามเรื่องปัญหาการใช้ Cookies บน Server คับผม ขอความกรุณาด้วย

            <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 pmeters = "tUsername=" + encodeURI( document.getElementById("txtUsername").value) +
						"&tPassword=" + encodeURI( document.getElementById("txtPassword").value );
		  var _chk = document.getElementById("chk").value;
		  if(document.frmMain.chk.checked == false){
			  var url = 'chklogin.php';
			  }

		  if(document.frmMain.chk.checked == true){
			  var url = 'chklogin.php?chk=on';
			}

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

	   }
	</script>
            <form name="frmMain">

                       <input name="txtUsername" id="txtUsername"  type="text"  class="name" style="width:270px; height:36px;" onKeyUp="replaceDigit(this)" placeholder="[email protected]" value="<?=$_SESSION["Mem_Mail"]?>" />

                      <input name="txtPassword" id="txtPassword" type="password"  class="name" style="width:270px; height:36px;" onKeyUp="replaceDigit(this)" placeholder="Password"  /></div></th>
				จำฉันไว้ในระบบ		<input type="checkbox" name="chk" id="chk" value="on"/>
             <input name="btnLogin" type="button" class="bl" id="btnLogin" OnClick="JavaScript:doCallAjax();"></td>

          </form>