add_submenu_page()

function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function = '', $position = null ) {
	global $submenu, $menu, $_gc_real_parent_file, $_gc_submenu_nopriv,
		$_registered_pages, $_parent_pages;

	$menu_slug   = plugin_basename( $menu_slug );
	$parent_slug = plugin_basename( $parent_slug );

	if ( isset( $_gc_real_parent_file[ $parent_slug ] ) ) {
		$parent_slug = $_gc_real_parent_file[ $parent_slug ];
	}

	if ( ! current_user_can( $capability ) ) {
		$_gc_submenu_nopriv[ $parent_slug ][ $menu_slug ] = true;
		return false;
	}

	/*
	 * If the parent doesn't already have a submenu, add a link to the parent
	 * as the first item in the submenu. If the submenu file is the same as the
	 * parent file someone is trying to link back to the parent manually. In
	 * this case, don't automatically add a link back to avoid duplication.
	 */
	if ( ! isset( $submenu[ $parent_slug ] ) && $menu_slug !== $parent_slug ) {
		foreach ( (array) $menu as $parent_menu ) {
			if ( $parent_menu[2] === $parent_slug && current_user_can( $parent_menu[1] ) ) {
				$submenu[ $parent_slug ][] = array_slice( $parent_menu, 0, 4 );
			}
		}
	}

	$new_sub_menu = array( $menu_title, $capability, $menu_slug, $page_title );
	if ( ! is_int( $position ) ) {
		if ( null !== $position ) {
			_doing_it_wrong(
				__FUNCTION__,
				sprintf(
					/* translators: %s: add_submenu_page() */
					__( 'The seventh parameter passed to %s should be an integer representing menu position.' ),
					'<code>add_submenu_page()</code>'
				),
				'5.3.0'
			);
		}

		$submenu[ $parent_slug ][] = $new_sub_menu;
	} else {
		// Append the submenu if the parent item is not present in the submenu,
		// or if position is equal or higher than the number of items in the array.
		if ( ! isset( $submenu[ $parent_slug ] ) || $position >= count( $submenu[ $parent_slug ] ) ) {
			$submenu[ $parent_slug ][] = $new_sub_menu;
		} else {
			// Test for a negative position.
			$position = max( $position, 0 );
			if ( 0 === $position ) {
				// For negative or `0` positions, prepend the submenu.
				array_unshift( $submenu[ $parent_slug ], $new_sub_menu );
			} else {
				// Grab all of the items before the insertion point.
				$before_items = array_slice( $submenu[ $parent_slug ], 0, $position, true );
				// Grab all of the items after the insertion point.
				$after_items = array_slice( $submenu[ $parent_slug ], $position, null, true );
				// Add the new item.
				$before_items[] = $new_sub_menu;
				// Merge the items.
				$submenu[ $parent_slug ] = array_merge( $before_items, $after_items );
			}
		}
	}
	// Sort the parent array.
	ksort( $submenu[ $parent_slug ] );

	$hookname = get_plugin_page_hookname( $menu_slug, $parent_slug );
	if ( ! empty( $function ) && ! empty( $hookname ) ) {
		add_action( $hookname, $function );
	}

	$_registered_pages[ $hookname ] = true;

	/*
	 * Backward-compatibility for plugins using add_management_page().
	 * See gc-admin/admin.php for redirect from edit.php to tools.php.
	 */
	if ( 'tools.php' === $parent_slug ) {
		$_registered_pages[ get_plugin_page_hookname( $menu_slug, 'edit.php' ) ] = true;
	}

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

	return $hookname;
}

/**
 * Adds a submenu page under a custom post type parent.
 */
function books_register_ref_page() {
    add_submenu_page(
        'edit.php?post_type=book',
        __( 'Books Shortcode Reference', 'textdomain' ),
        __( 'Shortcode Reference', 'textdomain' ),
        'manage_options',
        'books-shortcode-ref',
        'books_ref_page_callback'
    );
}

/**
 * Display callback for the submenu page.
 */
function books_ref_page_callback() { 
    ?>
    <div class="wrap">
        <h1><?php _e( 'Books Shortcode Reference', 'textdomain' ); ?></h1>
        <p><?php _e( 'Helpful stuff here', 'textdomain' ); ?></p>
    </div>
    <?php
}

/**
 * Sub menu class
 *
 * @author Mostafa <mostafa.soufi@hotmail.com>
 */
class Sub_menu {

	/**
	 * Autoload method
	 * @return void
	 */
	public function __construct() {
		add_action( 'admin_menu', array(&$this, 'register_sub_menu') );
	}

	/**
	 * Register submenu
	 * @return void
	 */
	public function register_sub_menu() {
		add_submenu_page( 
			'options-general.php', 'Submenu title', 'Submenu title', 'manage_options', 'submenu-page', array(&$this, 'submenu_page_callback')
		);
	}

	/**
	 * Render submenu
	 * @return void
	 */
	public function submenu_page_callback() {
		echo '<div class="wrap">';
		echo '<h2>Submenu title</h2>';
		echo '</div>';
	}

}

new Sub_menu();

add_action('admin_menu', 'gcdocs_register_my_custom_submenu_page');

function gcdocs_register_my_custom_submenu_page() {
	add_submenu_page(
		'tools.php',
		'My Custom Submenu Page',
		'My Custom Submenu Page',
		'manage_options',
		'my-custom-submenu-page',
		'gcdocs_my_custom_submenu_page_callback' );
}

function gcdocs_my_custom_submenu_page_callback() {
	echo '<div class="wrap"><div id="icon-tools" class="icon32"></div>';
		echo '<h2>My Custom Submenu Page</h2>';
	echo '</div>';
}

Class GCDocs_AdminPage
{
    private $_plugin_name;

    private $_version;

    public function __construct( $plugin_name, $version )
    {
        $this->_plugin_name = $plugin_name;
        $this->_version = $version;
    }

    public function gcdocs_create_menu_and_submenu_page()
    {
        add_menu_page(
            'PAGE TITLE', 'MENU TITLE', 'CAPABILITY', 'menu_slug',
            array( $this, 'gcdocs_method_name_in_the_class' ), 'icon_url', 'POSITION'
        );
        add_submenu_page(
            'parent_slug', 'PAGE TITLE', 'MENU TITLE', 'CAPABILITY', 'menu_slug', 
            array( $this, 'gcdocs_method_name_in_the_class' )
        );
    }
}
$admin_page = new GCDocs_AdminPage;
add_action( 'admin_menu', array( $admin_page, 'gcdocs_create_menu_and_submenu_page' ) );