Dynamic Multilevel Menu for Bootstrap

/* Dynamic Multilevel Menu (Implement the recursion logic)
 * Tutorial: https://www.w3school.info/2015/12/22/steps-to-create-dynamic-multilevel-menu-using-php-and-mysql/
 */
function get_menu_tree($parent_id) {
    global $con;
    $menu = "";
    $nav_arrow = "";
    $sqlquery = " SELECT * FROM menu WHERE status='1' AND menu_type='main' AND parent_id='" .$parent_id . "' ";
    $res = mysqli_query($con, $sqlquery);
    while($row = mysqli_fetch_array($res, MYSQLI_ASSOC)) {
        $menuid = $row['menu_id'];
        $menutree = get_menu_tree($row['menu_id']);
        if($menutree) {
            $nav_arrow = " class='dropdown'";
            $nav_tog = " class='dropdown-toggle' data-toggle='dropdown' role='button' aria-haspopup='true' aria-expanded='false'";
            $caret = " <span class='caret'></span>";
            $drop_menu = " class='dropdown-menu'";
        } else {$nav_arrow = $nav_tog = $caret = $drop_menu = ""; }
        if($menuid != 1) { $sronly = ""; } else { $sronly = " <span class='sr-only'>(current)</span>"; }
        $menu .="<li$nav_arrow><a href='".$row['link']."'$nav_tog>".$row['menu_name'].$sronly.$caret;
        if($menutree) {
            $menu .= "</a><ul$drop_menu>".get_menu_tree($row['menu_id'])."</ul>"; //call  recursively
        } else {
            $menu .= "</a>";
        }
        $menu .= "</li>";
    }
    return $menu;
}