The function below is intended to return a list of the WordPress submenu pages.

function get_admin_menus() {
    global $submenu, $menu, $pagenow;
    if ( current_user_can('manage_options') ) {
        if( $pagenow == 'index.php' ) {
        foreach ( $submenu as $index => $menu_item ) {
            foreach ( $menu_item as $value) {
      
                    $submenu_page = $value[2];
                
                    // return $submenu_page;
                    echo $submenu_page;

                }
            }
        }
    }
}

Adding the action add_action( 'admin_notices', 'get_admin_menus' ); displays a list of the submenu items. This only works with echo.

I would like to return the values into another function however when using return $submenu_page with echo get_admin_menus() does not appear to work.

For example:

function test () {
  echo get_admin_menus();
}

What am I missing or doing wrong?