[PHP] มีปัญหาการแสดงผล โดยใช้คำสั่ง .ResponseText; ของ Ajax

<script language="JavaScript">
	
		function checkbrowser(){
			var 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;
			}
			return HttPRequest;
		}
			
		function Adduser(mode,divid) {
			HttPRequest = checkbrowser();
			
			
				var url = 'insertuser.php';
				var pmeters = "username=" + encodeURI( document.getElementById("newuser").value) +
								"&mode=" + mode +
								"&password=" + encodeURI( document.getElementById("newpass").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(divid).innerHTML = "Now is Loading...";
							}
						if(HttPRequest.readyState == 4 && HttPRequest.readyState == 200) // Return Request
							{
								document.getElementById(divid).innerHTML = HttPRequest.responseText;
							}
						
					}
		}
</script>		
		
		<form id="form1" name="form1">
		            <table border="0" cellpadding="3px">
		              <tr>
		                <td width="23%">Username</td>
		                <td width="34%"><input id="newuser" name="newuser" type="text" width="100px"/></td>
		              	<td width="43%" rowspan="2"><input id="submit" name="submit" type="submit" value="ADD NEW USER" OnClick="JavaScript:Adduser('add','sta');"/><div id ="sta">.</div></td>
		              </tr>
		              <tr>
		                <td>Password</td>
		                <td><input id="newpass" name="newpass" type="text" width="100px" /></td>
		              </tr>
		            </table>
		            </form>

<?php 
	$username=$_POST['username'];
	$password=$_POST['password'];
	$strmode = $_POST["mode"];
		
		if($strmode=='add'){
			if($username!='' && $password!=''){
				$sql="insert into member set
						user = '$username',
						pass = '$password'";
				$result=mysql_query($sql) or exit($sql);
				
				echo "Successfull";
			}
		}
?>