Kostya

<script>
function doload(value){
var req=new JsHttpRequest();
req.onreadystatechange=function(){
if(req.readyState==4) document.getElementById("rub2").innerHTML=req.responseText;}
req.open(null,"select2.php",true);
req.send({rub1:value});}
</script>
<script>
function doload2(value){
var req=new JsHttpRequest();
req.onreadystatechange=function(){
if(req.readyState==4) document.getElementById("rub3").innerHTML=req.responseText;}
req.open(null,"select3.php",true);
req.send({rub2:value});}
</script>

<form action="" method="POST">

res=mysql_query('SELECT * FROM `SEARCH_rub1` order by `name`');
	$str='';
	$tpl=file_get_contents('template/search_item.tpl');
	while ($a=mysql_fetch_assoc($res))
	{
		$str.=str_replace(array('{VAL}','{TEXT}'),array($a['id'],$a['name']),$tpl);
	}
	$tpl2=file_get_contents('template/search_form.tpl');
	$html='';
	$html.=str_replace('{SELECT_ITEM}',$str,$tpl2);
	echo $html;

<div id="rub2"></div>
<div id="rub3"></div>
<input type="reset">

require("JsHttpRequest.php"); //Подключаем библиотеку
$JsHttpRequest=new JsHttpRequest("utf-8"); //Создаем экземпляр класса, указываю рабочую кодировку.
include_once('modules/db.php');
mysql_query("set names utf8");
$res=mysql_query('select * from `SEARCH_rub2` where `id_rub1`="'.intval($_REQUEST['rub1']).'" order by `name`');
$str='';
	$tpl=file_get_contents('template/search_item.tpl');
	while ($a=mysql_fetch_assoc($res))
	{
		$str.=str_replace(array('{VAL}','{TEXT}'),array($a['id'],$a['name']),$tpl);
	}
	$tpl2=file_get_contents('template/search_form2.tpl');
	$str2='';
	$str2.=str_replace('{SELECT_ITEM}',$str,$tpl2);
	echo $str2;

require("JsHttpRequest.php"); //Подключаем библиотеку
$JsHttpRequest=new JsHttpRequest("utf-8"); //Создаем экземпляр класса, указываю рабочую кодировку.
include_once('modules/db.php');
mysql_query("set names utf8");
$res=mysql_query('select * from `SEARCH_rub3` where `id_rub2`='.$_REQUEST['rub2']);
	$str='';
	$tpl=file_get_contents('template/search_item.tpl');
	while ($a=mysql_fetch_assoc($res))
	{
		$str.=str_replace(array('{VAL}','{TEXT}'),array($a['id'],$a['name']),$tpl);
	}
	$tpl2=file_get_contents('template/search_form3.tpl');
	$str2='';
	$str2.=str_replace('{SELECT_ITEM}',$str,$tpl2);
	echo $str2;

function doload(value){
var req=new JsHttpRequest();
req.onreadystatechange=function(){
if(req.readyState==4) document.getElementById("rub2").innerHTML=req.responseText;}
req.open(null,"select2.php",true);
req.send({rub1:value});}