add_submenu_page( string $parent_slug, string $page_title, string $menu_title, string $capability, string $menu_slug, callable $callback = '', int|float $position = null )

function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function = '' ) {
	global $submenu, $menu, $_wp_real_parent_file, $_wp_submenu_nopriv,
		$_registered_pages, $_parent_pages;

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

	if ( isset( $_wp_real_parent_file[$parent_slug] ) )
		$parent_slug = $_wp_real_parent_file[$parent_slug];

	if ( !current_user_can( $capability ) ) {
		$_wp_submenu_nopriv[$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 );
		}
	}

	$submenu[$parent_slug][] = array ( $menu_title, $capability, $menu_slug, $page_title );

	$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 wp-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;
}