Populate triple drop down list from database using Ajax and PHP

im looking for populate drop down in php..b4 this im using dotnet its easy just drag and added a simple code..but in php we have to understant javascrpt and php..after google a while this is the easy code to populate the drop down....copy from here...
___________________

TABLE

CREATE TABLE `country` (
`id` tinyint(4) NOT NULL auto_increment,
`country` varchar(20) NOT NULL default '',
PRIMARY KEY (`id`)
) TYPE=MyISAM ;

CREATE TABLE `state` (
`id` tinyint(4) NOT NULL auto_increment,
`countryid` tinyint(4) NOT NULL,
`statename` varchar(40) NOT NULL,
PRIMARY KEY (`id`)
) TYPE=MyISAM ;

CREATE TABLE `city` (
`id` tinyint(4) NOT NULL auto_increment,
`city` varchar(50) default NULL,
`stateid` tinyint(4) default NULL,
`countryid` tinyint(4) NOT NULL,
PRIMARY KEY (`id`)
) TYPE=MyISAM ;

______________________

index.php

Country	Select CountryUSACanada
State	Select Country First
City	Select State First

_____________________

function getState(countryId)
{
var strURL="findState.php?country="+countryId;
var req = getXMLHTTP();
if (req)
{
req.onreadystatechange = function()
{
if (req.readyState == 4)
{
// only if "OK"
if (req.status == 200)
{
document.getElementById('statediv').innerHTML=req.responseText;
} else {
alert("There was a problem while using XMLHTTP:\n" + req.statusText);
}
}
}
req.open("GET", strURL, true);
req.send(null);
}
}

_______
findState.php


$link = mysql_connect('localhost', 'root', ''); /change the configuration if required
if (!$link) {
die('Could not connect: ' . mysql_error());
}
mysql_select_db('db_ajax'); //change this if required
$query="select city from city where countryid=$country";
$result=mysql_query($query);?>

Select City