AJAX Question

Hi,

I am currently using this to control a country and region drop down. So when i select a country the region box will fill with the results automatically.

// JavaScript Document

	var AdminResponse = "";

	function parseResponse(){

		var nText = AdminResponse.getElementsByTagName('optionText');
		var nVal = AdminResponse.getElementsByTagName('optionVal');
		document.forms[0]['regionid'].options.length = 1;
		for (i=0; i<nText.length; i++)
			{ 
			 var nOption = document.createElement('option'); 
			 var isText = document.createTextNode(nText[i].firstChild.data); 
			 nOption.setAttribute('value',nVal[i].firstChild.data); 
			 nOption.appendChild(isText); 
			 document.forms[0]['regionid'].appendChild(nOption); 
			}
	}

	function update(nVal){

		var AdminRequest = window.ActiveXObject ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest();   
		AdminRequest.onreadystatechange = function()
			{
		 	 if (AdminRequest.readyState == 4)
				{
		 	 	 if (AdminRequest.status == 200)
					{
			 	 	 AdminResponse = AdminRequest.responseXML;
			 	 	 parseResponse();
					}
		 	 	 else 	{
				 	 alert('Error Update.php File '+ AdminRequest.statusText);
					}
				}
			}
		var infoStr = "?choice="+nVal;
		AdminRequest.open("GET", "Update.php"+infoStr, true);
		AdminRequest.send(null); 
	}

Code (markup):

<?php 

	$choice = $_GET['choice'];
	$xml = "<?xml version='1.0' ?><options>";

	include ("../config/db.config.php");

	$query = "SELECT * FROM regions WHERE countryid = '$choice'";
	$result = @mysql_query($query);
	$num = @mysql_num_rows($result);
	if ($result && $num > 0)
		{
	 	 while ($row = mysql_fetch_array($result,MYSQL_ASSOC))
			{
	 	 	 $xml .= "<optionText>" . $row['region'] . "</optionText><optionVal>" . $row['id'] . "</optionVal>";
			}
		}
        $xml .= "</options>";
	@mysql_free_result($result);
	@mysql_close();	
	header("Content-Type: text/xml");
	echo $xml;

?>

PHP:

This works fine but now i want to include a third box for cities so when a region is selected the the cities dropdown will automatically fill.

How can i do it with the current code?

Cheers,
Adam