Wordpress admin panel menu and sub menu

wp_menu_with_submenu.php

<?php

function registration_menu()
{   
  add_menu_page( 'Registration', 'Registration', 'edit_posts', 'registration', 'display_registration_page', 'dashicons-admin-network' );
  add_submenu_page( 'registration', 'Registration Settings', 'Settings', 'edit_posts', 'registration_settings', 'page_callback_function1' );
}

function display_registration_page(){
	ob_start();
    require_once REG_VIEW_PATH . 'content-registration.php'; 
	$content = ob_get_contents();
	return $content;
}

add_action('admin_menu', 'registration_menu');