Bagaimana untuk Mencipta Menu Dropdown Bergantung Menggunakan PHP dan AJAX?

Table: category
| id | master | name |
| --- | ------ | ----- |
| 1   | 0 | Main 1 |
| 2   | 1 | Sub 1 |
| 3   | 1 | Sub 2 |
| 4   | 0 | Main 2 |
| 5   | 4 | Sub 3 |
| 6   | 4 | Sub 4 |

<select name="master">

<?php
if (isset($_POST['master_id']) && $_POST['master_id'] != '') {
    $master_id = $_POST['master_id'];
    $sql = "SELECT * FROM `category` WHERE `master` = ?";
    $statement = $objDb->prepare($sql);
    $statement->execute(array($master_id));
    $list = $statement->fetchAll(PDO::FETCH_ASSOC);
    echo '<option value="" selected disabled>Select Sub</option>';
    if (!empty($list)) {
        foreach ($list as $row) {
            echo '<option value="' . $row['id'] . '">' . $row['name'] . '</option>';
        }
    }
}
?>