add_menu_page()

function add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function = '', $icon_url = '', $position = null ) {
	global $menu, $admin_page_hooks, $_registered_pages, $_parent_pages;

	$menu_slug = plugin_basename( $menu_slug );

	$admin_page_hooks[ $menu_slug ] = sanitize_title( $menu_title );

	$hookname = get_plugin_page_hookname( $menu_slug, '' );

	if ( ! empty( $function ) && ! empty( $hookname ) && current_user_can( $capability ) ) {
		add_action( $hookname, $function );
	}

	if ( empty( $icon_url ) ) {
		$icon_url   = 'dashicons-admin-generic';
		$icon_class = 'menu-icon-generic ';
	} else {
		$icon_url   = set_url_scheme( $icon_url );
		$icon_class = '';
	}

	$new_menu = array( $menu_title, $capability, $menu_slug, $page_title, 'menu-top ' . $icon_class . $hookname, $hookname, $icon_url );

	if ( null === $position ) {
		$menu[] = $new_menu;
	} elseif ( isset( $menu[ "$position" ] ) ) {
		$position            = $position + substr( base_convert( md5( $menu_slug . $menu_title ), 16, 10 ), -5 ) * 0.00001;
		$menu[ "$position" ] = $new_menu;
	} else {
		$menu[ $position ] = $new_menu;
	}

	$_registered_pages[ $hookname ] = true;

	// No parent as top level.
	$_parent_pages[ $menu_slug ] = false;

	return $hookname;
}

/**
 * Register a custom menu page.
 */
function gcdocs_register_my_custom_menu_page() {
	add_menu_page(
		__( 'Custom Menu Title', 'textdomain' ),
		'custom menu',
		'manage_options',
		'myplugin/myplugin-admin.php',
		'',
		plugins_url( 'myplugin/images/icon.png' ),
		6
	);
}
add_action( 'admin_menu', 'gcdocs_register_my_custom_menu_page' );

/**
 * Register a custom menu page.
 */
function gcdocs_register_my_custom_menu_page(){
	add_menu_page( 
		__( 'Custom Menu Title', 'textdomain' ),
		'custom menu',
		'manage_options',
		'custompage',
		'my_custom_menu_page',
		plugins_url( 'myplugin/images/icon.png' ),
		6
	); 
}
add_action( 'admin_menu', 'gcdocs_register_my_custom_menu_page' );

/**
 * Display a custom menu page
 */
function my_custom_menu_page(){
	esc_html_e( 'Admin Page Test', 'textdomain' );	
}

/**
 * Create admin Page to list unsubscribed emails.
 */
 // Hook for adding admin menus
 add_action('admin_menu', 'gcdocs_unsub_add_pages');

 // action function for above hook

/**
 * Adds a new top-level page to the administration menu.
 */
function gcdocs_unsub_add_pages() {
     add_menu_page(
	 	__( 'Unsub List', 'textdomain' ),
		__( 'Unsub Emails','textdomain' ),
		'manage_options',
		'gcdocs-unsub-email-list',
		'gcdocs_unsub_page_callback',
		''
	);
}

/**
 * Disply callback for the Unsub page.
 */
 function gcdocs_unsub_page_callback() {
     echo 'Unsubscribe Email List';
 }
 

class GCDocs_EB_EbtechModules {
    public static function init() {
        add_action( 'admin_enqueue_scripts', array( __CLASS__, 'adminAssets' ) );
        add_action( 'admin_menu', array( __CLASS__, 'adminMenu' ) );
    }

    public static function adminMenu() {
        add_menu_page(
            __( 'GCDocs Ebtech Menu', 'gcdocs-webnail-modules' ),
            __( 'GCDocs Ebtech modules', 'gcdocs-webnail-modules' ),
            'manage_options',
            'gcdocs-ebtech-modules',
            array( __CLASS__, 'menuPage' ),
            'dashicons-tagcloud',
            6
        );
    }

    public static function menuPage() {
        if ( is_file( plugin_dir_path( __FILE__ ) . 'includes/layout.php' ) ) {
            include_once plugin_dir_path( __FILE__ ) . 'includes/layout.php';
        }
    }

    public static function getSettings() {
        return get_option( 'gcdocs_ebtech_modules_option' );
    }

    public static function adminAssets() {
        if ( isset( $_GET['page'] ) && ! empty( $_GET['page'] ) && 'gcdocs-ebtech-modules' === $_GET['page'] ) {

        }
    }
}

GCDocs_EB_EbtechModules::init();
GCDocs_EB_EbtechModules::getSettings();

// For those who are object orientated. Add a class 
// function as the menu callback and setup the 
// menus automatically. 

// Exit if accessed directly
if ( !defined( 'ABSPATH' ) ) exit;

class MyMenuSetterUpper {

    private static $instance;

    /**
     * Main Instance
     *
     * @staticvar 	array 	$instance
     * @return 		The one true instance
     */
    public static function instance() {
         if ( ! isset( self::$instance ) ) {
             self::$instance = new self;
	     self::$instance->addMyAdminMenu();
	}

        return self::$instance;
    }

    public function addMyAdminMenu() {
         
         add_menu_page(
            'My Page Title',
            'My Page',
            'read',
            'my-menu-page-slug',
            array(
                $this,
                'myAdminPage'
            ),
            'to/icon/file.svg',
            '2.1'
        );
    }

    public function myAdminPage() {
         // Echo the html here...
    }

}
// Call the class and add the menus automatically. 
$MyMenuSetterUpper = MyMenuSetterUpper::instance();