ajax issue in zencart - php

Question

I have some main categories and subcategories of each main category..I've a dropdown list contains main category, when i choose a main category then the subcategory dropdown list shows the subcategories of that main category..
am using the following code for this, but this shows the subcategory box contains the whole page with header and footer...
<select name="main_category" id="main_category" onchange="showSubCategory(this.value)">
<option>--Select--</option>
</select>
<script type="text/javascript">
function showSubCategory(str)
{
if (str.length==0)
{
document.getElementById("txtHint").innerHTML="";
return;
}
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
alert(xmlhttp.responseText);
document.getElementById("subcategory").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","index.php?main_page=getsubcategory&cid="+str,true);
xmlhttp.send();
}
</script>
in tpl_subcategory_default.php contains
<?php
$cid=$_GET['cid'];
$sql="select cd.categories_name, cd.categories_id
from " . TABLE_CATEGORIES . " c, " . TABLE_CATEGORIES_DESCRIPTION . " cd
where c.parent_id = '" . (int) $_GET['cid'] . "'
and c.categories_id = cd.categories_id
and c.categories_status= 1";
$r=mysql_query($sql);
while($row=mysql_fetch_array($r))
{
echo "<option value=$row[categories_id]>$row[categories_name]</option>";
}
?>

Answer 1

It's showing the whole page with header and footer because you're accessing the "page" via index.php?main_page=foo but haven't added architecture to replace the normal templating system outputs with your own page-specific output ... ie: to jump directly to the output without first calling the normal things which appear on every page.
Your question can't really be accurately answered without knowing also what you've done in the /includes/modules/pages/subcategory/header_php.php file, or if you've even created one. Chances are that the code you've put into tpl_subcategory_default.php could go into the header_php.php file mentioned above, followed by a die() statement at the end, and accomplish the same thing which you seem to be looking for.
It would be easier to answer your question fully if you supplied more information about what you've done so far.

Answer 2

To remove header,footer etc you can override the tpl_main_page.php. Go to this directory /includes/templates/custom template. As per your information you have created main_page=getsubcategory page. so create a folder named getsubcategory under this directory. then copy tpl_main_page.php from includes/templates/custom template/common/ and paste it in /includes/templates/your custom template/getsubcategory. Then do the below changes in tpl_main_page.php file.
if (in_array($current_page_base,explode(",",'getsubcategory')) ) {
$flag_disable_left = true;
$flag_disable_header = true;
$flag_disable_footer = true;
}