<?php
$con = mysqli_connect("localhost","root","","imetrics") or die ("Cannot connect to the database!");

?>

    <form name="form1" action="" method="post">
        <table>
            <tr>
                <td>Select Category</td>
                <td><select class="form-control" style="width: 150px;" id="categorydd" name="catdd" onChange="change_category()">
                        <option>Select</option>
                        <?php
                            $query=mysqli_query($con, "SELECT category_id, categoryname FROM category WHERE ParentCategoryID IS NULL");
                            while($row=mysqli_fetch_array($query)) {
                                ?>
                                <option value="<?php echo $row["category_id"]; ?>"><?php echo $row["categoryname"]; ?></option>

                                <?php
                            }
                        ?>




                    </select>
                </td>
            </tr>

            <tr>
                <td>Select Subcategory</td>
                <td>
                    <div id="subcategory">
                        <select class="form-control" style="width: 150px;">
                            <option>Select</option>
                        </select>
                    </div>
                </td>
            </tr>

            <tr>
                <td>Select Question</td>
                <td>
                    <div id="question">
                        <select class="form-control" style="width: 150px;">
                            <option>Select</option>
                        </select>
                    </div>
                </td>
            </tr>

        </table>


    </form>

<script type="text/javascript">

        function change_category()
        {
            var xmlhttp = new XMLHttpRequest();
            xmlhttp.open("GET","new2.php?category="+document.getElementById("categorydd").value, false);
            xmlhttp.send(null);
            document.getElementById("subcategory").innerHTML = xmlhttp.responseText;

            if(document.getElementById("categorydd").value=="Select"){
                document.getElementById("question").innerHTML="<select><option>Select</option></select>";
            }
        }


    function change_question()
    {
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("GET","new2.php?subcategory=" + document.getElementById("subcategorydd").value, false);
        xmlhttp.send(null);
        document.getElementById("question").innerHTML = xmlhttp.responseText;
    }


</script>