Giúp đỡ về DropDown

<table border=0 cellpadding=0 cellspacing="0" align="center" width="40%">
	<form>		
		<tr><td colspan="2" height="25"></td></tr>		
		<tr><td colspan="2" class="textheading">AJAX Dropdown to Dropdown</td></tr>
		<tr><td colspan="2" height="25"></td></tr>		
		<tr><td class="text" width="50%">Select category : </td><td>
				<?php
					require_once("get.php");
					$obj = new AjaxDropdown();
					$arr = $obj->getArray(0);
					$strRet = '<option value="0">--Select--</option>';
					foreach ( $arr as $row )
					{
						$strRet .= '<option value="'.$row->id.'">'.$row->Name.'</option>';
					}					
				?>
				<select name="selCat" class="text" onChange="javascript:Ajax.Request('get.php?method=getXML&param=',this.value, Ajax.Response);">
					<?php echo $strRet; ?>
				</select>
				</td></tr>
				<tr><td colspan="2" height="5"></td></tr>
				<tr><td class="text">Sub categories :</td><td>
				<select name="selSubCat" id="selSubCat" class="text">
					<option value="0">--Select--</option>
					<option value="12">test</option>
				</select>
				</td></tr>
	</form>
	</table>

<?php

DEFINE ('DB_USER', 'root');
DEFINE ('DB_PASSWORD', '');
DEFINE ('DB_HOST', 'localhost');
DEFINE ('DB_NAME', 'testdb');

class AjaxDropdown
{
	var $table;
	
	function AjaxDropdown()
	{		
		// Make the connnection and then select the database.
		$dbc = @mysql_connect (DB_HOST, DB_USER, DB_PASSWORD) OR die ('Could not connect to MySQL: ' . mysql_error() );
		mysql_select_db (DB_NAME) OR die ('Could not select the database: ' . mysql_error() );
		$this->table = "tblcategories";
	}
	
	function dbConnect()
	{
		DEFINE ('LINK', mysql_connect (DB_HOST, DB_USER, DB_PASSWORD));
	}
	
	function getXML($id)
	{
		$this->dbConnect();
		$query = "SELECT * FROM $this->table where pid = {$id} ORDER BY id asc";
		$result = mysql_db_query (DB_NAME, $query, LINK);
		
		$xml = '<?xml version="1.0" encoding="ISO-8859-1" ?>';
		$xml .= '<categories>';
		while($row = mysql_fetch_array($result))
		{
			$xml .= '<category>';
			$xml .= '<id>'. $row['id'] .'</id>';
			$xml .= '<fname>'. $row['Name'] .'</fname>';
			$xml .= '</category>';
		}
		$xml .= '</categories>';
		mysql_close();		
		return $xml;
	}	
	
	function getArray($id)
	{
		$this->dbConnect();
		$query = "SELECT * FROM $this->table where pid = {$id} ORDER BY id asc";
		$result = mysql_db_query (DB_NAME, $query, LINK);
		$arr = array();
		while($row = mysql_fetch_object($result))
		{
			$arr[] = $row;
		}
		mysql_close();		
		return $arr;	
	}
}

if ( @$_GET['method'] == 'getXML' )
{
	header("Content-Type: application/xml; charset=UTF-8");
	$obj = new AjaxDropdown();
	echo $obj->getXML(@$_GET['param']);
}

var Ajax = new Object();

Ajax.Request = function(url,id, callbackMethod)
{
	if ( id == 0 )
	{
		return;
	}
	Page.getPageCenterX();

	Ajax.request = Ajax.createRequestObject();
	Ajax.request.onreadystatechange = callbackMethod;
	Ajax.request.open("POST", url+id, true);
	Ajax.request.send(url);
}

Ajax.Response = function ()
{
	if(Ajax.CheckReadyState(Ajax.request))
	{		
		document.getElementById('selSubCat').length = 0;
		document.getElementById('selSubCat').options[0] = new Option("--Select--","0");
		
		var	response = Ajax.request.respon***ML.documentElement;
		var _data = response.getElementsByTagName('category');
		if(_data.length == 0)
		{
			document.getElementById('selSubCat').options[0] = new Option("No sub category available",'0');	
		}
		var i
		for ( i = 0 ; i < _data.length ; i ++ )
		{
			document.getElementById('selSubCat').options[i] = new Option(response.getElementsByTagName('fname')[i].firstChild.data,response.getElementsByTagName('id')[i].firstChild.data);	
		}
	}
}

Ajax.createRequestObject = function()
{
	var obj;
	if(window.XMLHttpRequest)
	{
		obj = new XMLHttpRequest();
	}
	else if(window.ActiveXObject)
	{
		obj = new ActiveXObject("MSXML2.XMLHTTP");
	}
	return obj;
}

Ajax.CheckReadyState = function(obj)
{
	if(obj.readyState < 4) {		
		document.getElementById('loading').style.top = (Page.top + Page.height/2)-100;
		document.getElementById('loading').style.left = Page.width/2-75;
		document.getElementById('loading').style.position = "absolute";
		//document.getElementById('loading').innerHTML = "<table border=0 cellpadding=0 cellspacing=1 width=160 bgcolor=gray><tr><td align=center class=loading height=45 bgcolor=#ffffff><img src='snake_transparent.gif'/>&nbsp&nbspLoading..</td></tr></table>";  
	}
	//if(obj.readyState == 1) { document.getElementById('loading').innerHTML = "Loading..."; }
	//if(obj.readyState == 2) { document.getElementById('loading').innerHTML = "Loading..."; }
	//if(obj.readyState == 3) { document.getElementById('loading').innerHTML = "Loading..."; }	
	if(obj.readyState == 4)
	{
		if(obj.status == 200)
		{
			//document.getElementById('loading').innerHTML = "<table border=0 cellpadding=0 cellspacing=1 width=160 bgcolor=gray><tr><td align=center class=loaded height=45 bgcolor=#ffffff>Data is loaded successfully..</td></tr></table>";
			setTimeout('Page.loadOut()',2000);
			return true;
		}
		else
		{
			document.getElementById('loading').innerHTML = "HTTP " + obj.status;
		}
	}
}

var Page = new Object();
Page.width;
Page.height;
Page.top;

Page.loadOut = function ()
{
	document.getElementById('loading').innerHTML ='';	
}
Page.getPageCenterX = function ()
{
		var fWidth;
		var fHeight;		
		//For old IE browsers 
		if(document.all) 
		{ 
		fWidth = document.body.clientWidth; 
		fHeight = document.body.clientHeight; 
		} 
		//For DOM1 browsers 
		else if(document.getElementById &&!document.all)
		{ 
		fWidth = innerWidth; 
		fHeight = innerHeight; 
		} 
		else if(document.getElementById) 
		{ 
		fWidth = innerWidth; 
		fHeight = innerHeight; 		
		} 
		//For Opera 
		else if (is.op) 
		{ 
		fWidth = innerWidth; 
		fHeight = innerHeight; 		
		} 
		//For old Netscape 
		else if (document.layers) 
		{ 
		fWidth = window.innerWidth; 
		fHeight = window.innerHeight; 		
		}
	Page.width = fWidth;
	Page.height = fHeight;
	Page.top = window.document.body.scrollTop;
}

create database testdb;

use testdb;

CREATE TABLE `tblcategories` (
  `id` int(11) NOT NULL,
  `Name` varchar(100) collate latin1_general_ci NOT NULL,
  `pid` int(11) NOT NULL
);