com_dtm_cart.php

    <?php
    defined( '_JEXEC' ) or die( 'Restricted access' );
    $doc = JDocument::getDocument();
    $js ='
    function showUser(str) {
    var xmlhttp;


           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) {
                   document.getElementById("txtHint").innerHTML = xmlhttp.responseText;
               }
           }

           xmlhttp.open("GET","option=com_dtm_cart=myQuery?q="+str,true);
           xmlhttp.send();

    }
    ';
    $doc->addScriptDeclaration($js);
    require_once (dirname(__FILE__).'/controller.php');


    controller.php


   <?php
jimport('joomla.application.component.controller');


class dmtController extends JController
{

        function myQuery {
            $doc = JFactory::getDocument();
            $db= JFactory::getDbo();
            $ready = $db->getQuery(true);
            $q = htmlspecialchars($_REQUEST['q']);
            $query='SELECT category_name,virtuemart_category_id from #__virtuemart_categories_el_gr INNER JOIN #__virtuemart_category_categories ON #__virtuemart_categories_el_gr.virtuemart_category_id = #__virtuemart_category_categories.category_child_id WHERE #__virtuemart_category_categories.category_parent_id ='. $q;
            $db->setQuery($query);
            $options=$db->loadObjectList();
            $output="";
            foreach ($options as $row) {

                    $output .= '<option name='. $q .' value=' . $row->virtuemart_category_id . '>'. $row->category_name . '</option>';
                    }
            echo $output;
            }
}





    XML FILE:


    <?xml version="1.0" encoding="utf-8"?>
    <extension type="component" version="3.2" method="upgrade">

            <name>com_dtm_cart</name>
            <!-- The following elements are optional and free of formatting constraints -->
            <creationDate>December 2014</creationDate>
            <author>Thanasis kontokostas</author>
            <authorEmail>kontokostas.thanasis@gmail.com</authorEmail>
            <copyright>Copyright Info</copyright>
            <version>0.0.1</version>
            <description>Nothing to do here !!</description>
            <files>
                    <filename>controller.php</filename>
                    <filename>index.html</filename>
                    <filename>dtm_cart.php</filename>
                    <filename>scriptyo.js</filename>
                    <filename>dtm_cart.xml</filename>
                    <folder>tmpl</folder>
            </files>
     <administration>

                    <menu>YO</menu>

                    <files>
                    <filename>controller.php</filename>
                    <filename>index.html</filename>
                    <filename>dtm_cart.php</filename>
                    <filename>scriptyo.js</filename>
                    <filename>dtm_cart.xml</filename>
                    <folder>tmpl</folder>
                    </files>
            </administration>

    </extension>