<?php
    $query = "SELECT `menu_id`, `menu_name` FROM main_menu";
    if ($result = mysqli_query($conn,$query)) { // execute the rest of script only when query is executed successfully
            if (mysqli_num_rows($result) > 0) { // successfully executed query doesn't mean you must get the result
                WHILE ($row = mysqli_fetch_assoc($result)) {
                    // use echo instead of mixing html and script (easier to manage and easier to comment out
                    echo '<option value = "'.$num['menu_id'].'">'.$num['menu_name'].'</option>';
                }
            }
        }
 ?>