Two related drop down list by using Ajax & PHP

How dropdown list with Ajax script works?

1. main.php : Display the category listbox and posts Ajax request.
2. dd.php : Receives cat_id and collects matching subcategory from table.
3. mainck.php : Collects the form data and display the same.

main.php

<form name="testform" method='POST' action='mainck.php'>
Name:<input type=text name=fname>

require "config.php";// connection to database 

<?Php
require "config.php";// connection to database 

echo "<br>Select Category first  <select name=cat id='s1' onchange=AjaxFunction();>
<option value=''>Select One</option>";

$sql="select * from category "; // Query to collect data from table 

foreach ($dbo->query($sql) as $row) {
echo "<option value=$row[cat_id]>$row[category]</option>";
}
?>
</select>

echo "<br>Select Category first  <select name=cat id='s1' onchange=AjaxFunction();>

var cat_id=document.getElementById('s1').value;

	var url="dd.php";
var cat_id=document.getElementById('s1').value;
url=url+"?cat_id="+cat_id;
url=url+"&sid="+Math.random();
httpxml.onreadystatechange=stateck;
//alert(url);
httpxml.open("GET",url,true);
httpxml.send(null);

dd.php

if(!is_numeric($cat_id)){

echo "Data Error";

exit;

 }

$sql="select subcategory,subcat_id from subcategory where cat_id='$cat_id'";

$row=$dbo->prepare($sql);
$row->execute();
$result=$row->fetchAll(PDO::FETCH_ASSOC);

$main = array('data'=>$result);
echo json_encode($main);

var myarray = JSON.parse(httpxml.responseText);

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

for (i=0;i<myarray.data.length;i++)
{
var optn = document.createElement("OPTION");
optn.text = myarray.data[i].subcategory;
optn.value = myarray.data[i].subcat_id;  // You can change this to subcategory 
document.testform.subcat.options.add(optn);

} 

echo "\$_POST['fname'] = $_POST[fname]<br>
\$_POST['cat'] = $_POST[cat]<br>
\$_POST['subcat'] = $_POST[subcat]
";