hijole pues el ejemplo que te puse de la apagina ahi lo explican todo
pero bueno aqui te lo vuelvo a describir
archivo index.php
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<script type="text/javascript" src="ajax.js"></script>
<title>Documento sin título</title>
<style type="text/css">
body{
background-repeat:no-repeat;
font-family: Trebuchet MS, Lucida Sans Unicode, Arial, sans-serif;
height:100%;
background-color: #FFF;
margin:0px;
padding:0px;
}
select{
width:150px;
}
</style>
<script type="text/javascript" src="ajax.js"></script>
<script type="text/javascript">
/************************************************** ************************************************** ********
Ajax chained select
Copyright (C) 2006 DTHMLGoodies.com, Alf Magne Kalleland
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
Dhtmlgoodies.com., hereby disclaims all copyright interest in this script
written by Alf Magne Kalleland.
Alf Magne Kalleland, 2006
Owner of DHTMLgoodies.com
************************************************** ************************************************** ********/
var ajax = new sack();
function getCityList(sel)
{
var countryCode = sel.options[sel.selectedIndex].value;
document.getElementById('dhtmlgoodies_city').optio ns.length = 0; // Empty city select box
if(countryCode.length>0){
ajax.requestFile = 'getCities.php?countryCode='+countryCode; // Specifying which file to get
ajax.onCompletion = createCities; // Specify function that will be executed after file has been found
ajax.runAJAX(); // Execute AJAX function
}
}
function createCities()
{
var obj = document.getElementById('dhtmlgoodies_city');
eval(ajax.response); // Executing the response from Ajax as Javascript code
}
</script>
</head>
<body>
<form action="" method="post">
<table>
<tr>
<td>Country: </td>
<td><select id="dhtmlgoodies_country" name="dhtmlgoodies_country" onchange="getCityList(this)">
<option value="">Select</option>
<option value="dk">Denmark</option>
<option value="no">Norway</option>
<option value="us">US</option>
</select>
</td>
</tr>
<tr>
<td>City: </td>
<td><select id="dhtmlgoodies_city" name="dhtmlgoodies_city">
</select>
</td>
</tr>
</table>
</form>
</body>
</html>