Вот что я смог придумать, только сильно лажайте
select.php
<?php
ini_set('display_errors',1);
error_reporting(E_ALL ^E_NOTICE);
$db = mysql_connect ("localhost", "root", "");
mysql_query ("set character_set_client='cp1251'");
mysql_query ("set character_set_results='cp1251'");
mysql_query ("set collation_connection='cp1251_general_ci'");
mysql_select_db("cars",$db);
?>
<script src="JsHttpRequest.js"></script>
<script>
function doload(value){
var req=new JsHttpRequest();
req.onreadystatechange=function(){
if(req.readyState==4) document.getElementById("result").innerHTML=req.responseText;}
req.open(null,"select2.php",true);
req.send({brand:value});
}
function doload1(value){
var req=new JsHttpRequest();
req.onreadystatechange=function(){
if(req.readyState==4) document.getElementById("result1").innerHTML=req.responseText;}
req.open(null,"select3.php",true);
req.send({brand:value, model:value});
}
</script>
<form action="script.php" method="POST">
<select name="brand" id="brand" onchange="doload(this.value);">
<option value="no_action">–Выберите марку–</option>
<?php
$sql = mysql_query ("SELECT DISTINCT brand FROM t1");
while ($new_brand = mysql_fetch_array($sql))
echo '<option value='.$new_brand['brand'].'>'.$new_brand['brand'].'</option>';
?>
</select>
<br><br>
<div id="result"></div>
<br><br>
<div id="result1"></div>
<br><br>
<input type="submit" value="Отправить">
</form>
select2.php
<?php
$db = mysql_connect ("localhost", "root", "");
mysql_query ("set character_set_client='cp1251'");
mysql_query ("set character_set_results='cp1251'");
mysql_query ("set collation_connection='cp1251_general_ci'");
mysql_select_db("cars",$db);
require("JsHttpRequest.php");
$JsHttpRequest=new JsHttpRequest("windows-1251");
?>
<select name=\"model\" id=\"model\" onchange="doload2(this.value, this.value);">
<option value="no_action">–Выберите модель–</option>
<?
$sql = mysql_query ('SELECT DISTINCT model FROM t1 WHERE brand = "'.mysql_real_escape_string($_REQUEST["brand"]).'"');
if($sql) {
while ($new_model= mysql_fetch_array($sql)) {
echo '<option value="'.$new_model['model'].'">'.$new_model['model'].'</option>';
}
} else {
echo mysql_error();
}
?>
</select>
select3.php
<?php
$db = mysql_connect ("localhost", "root", "");
mysql_query ("set character_set_client='cp1251'");
mysql_query ("set character_set_results='cp1251'");
mysql_query ("set collation_connection='cp1251_general_ci'");
mysql_select_db("cars",$db);
require("JsHttpRequest.php");
$JsHttpRequest=new JsHttpRequest("windows-1251");
?>
<select name=\"body\" id=\"body\">
<option value="no_action">–Выберите тип кузова–</option>
<?
$sql = mysql_query ('SELECT DISTINCT body FROM t1 WHERE brand = "'.mysql_real_escape_string($_REQUEST["brand"]).'" AND model = "'.mysql_real_escape_string($_REQUEST["model"]).'"');
if($sql) {
while ($new_body= mysql_fetch_array($sql)) {
echo '<option value="'.$new_body['body'].'">'.$new_body['body'].'</option>';
}
} else {
echo mysql_error();
}
?>
</select>