<?php
     $conn = mysqli_connect("localhost", "root", "", "iraa");
 ?>
 Category :
<select  id="category_id" name="cat_id" required=""  class="form-control" tabindex="2" onselect ="getSubcat()">
    <option value="">-- Select --</option>
    <?php
        $sql = "SELECT category_id,category_name FROM categories_master";
        $result = mysqli_query($conn, $sql);
        while ($data=mysqli_fetch_array($result)) {
    ?>
    <option value="<?php echo $data['category_id'] ?>"><?php echo $data['category_name'] ?></option>
    <?php } ?>
</select>

Sub Category :
<select id="sub_cat_id" required  name="sub_cat_id"  class="form-control " tabindex="2">
    <option value="">-- Select --</option>
</select>

<script src="js/jquery-1.12.4.js"></script>
<script src="//rawgithub.com/indrimuska/jquery-editable-select/master/dist/jquery-editable-select.min.js"></script>
<link href="//rawgithub.com/indrimuska/jquery-editable-select/master/dist/jquery-editable-select.min.css" rel="stylesheet">
<script type="text/javascript">
    $('#category_id').editableSelect();
    function getSubcat() {
        var category_id=$("#category_id").val();
        alert(category_id);
        $.ajax({
            url: 'get_subcategory.php',
            type: 'POST',
            data: {category_id: category_id},
        })
        .done(function(response) {
            $('#sub_cat_id').html(response);
        });
    }
</script>

<?php
$conn = mysqli_connect("localhost", "root", "", "iraa");
$sql = "SELECT sub_category_id,sub_category_name FROM sub_categories_master WHERE category_id='$_POST[category_id]'";
$result = mysqli_query($conn, $sql);
while ($data=mysqli_fetch_array($result)) {
?>
<option value="<?php echo $data['sub_category_id'] ?>"><?php echo $data['sub_category_name'] ?></option>