sub-menu-page-createing

add_action('admin_menu', 'my_menu_pages');
function my_menu_pages(){
    add_menu_page('Order', 'Order', 'manage_options', 'order-page', 'order_output' );
    // add_submenu_page('my-menu', 'Submenu Page Title', 'Whatever You Want', 'manage_options', 'my-menu' );
    // add_submenu_page('my-menu', 'Submenu Page Title2', 'Whatever You Want2', 'manage_options', 'my-menu2' );
}