A WordPress plugin template with menu and submenu. Visit https://www.davidangulo.xyz/ for more information.

submenu.php

<?php
/*
Plugin Name: My WordPress Plugin
Plugin URI: https://wordpress.org/plugins/
Description: Just another WordPress plugin.
Version: 1.0.0
Author: David Angulo
Author URI: https://www.davidangulo.xyz/
*/

add_action('admin_menu', 'addMenuHooks');
function addMenuHooks(){
	add_menu_page( 'Appointments', 'Appointments', 'manage_options', 'appointments', 'mainMenu','dashicons-calendar-alt');
	add_submenu_page( 'appointments', 'Pending', 'Pending', 'manage_options', 'appointments-pending', 'subMenu' ); 
}

function mainMenu() {
	echo "<h2>My WordPress Plugin</h2>Hello world! I'm the main menu.";
}

function subMenu() {
	echo "<h2>My WordPress Plugin</h2>Hello world! I'm the submenu.";
}

//Visit the tutorial for more information
//https://www.davidangulo.xyz/website-development/how-to-add-submenu-in-wordpress-custom-plugin/