Three interlinked dropdown listbox using Ajax & PHP

By using JQuery we can create three linked dropdown list. This script is easy to understand and easy to modify.
Tutorial with Demo on using Three linked dropdown list - JQuery

First dropdown list: Country

Select Country<br><select name=country id='s1' onchange=ajaxFunction('s1');>
<option value=''>Select One</option>
<?Php
require "config.php";// connection to database 
$sql="select distinct country from student5 ";
foreach ($dbo->query($sql) as $row) {
echo "<option value=$row[country]>$row[country]</option>";
}
?>
</select>

Second dropdown list : State

for(j=document.myForm.state.options.length-1;j>=0;j--)
{
document.myForm.state.remove(j);
}

var state1=myObject.value.state;
for (i=0;i<myObject.state.length;i++)
{
var optn = document.createElement("OPTION");
optn.text = myObject.state[i];
optn.value = myObject.state[i];
document.myForm.state.options.add(optn);
if(optn.value==state1){
document.myForm.state.options[i].selected=true;
}
} 

City dropdown list : city

ajax-dd3ck.php