public static function get_item_ratings_returns() {

        return new external_single_structure(
            array(
                'ratings'    => new external_multiple_structure(
                    new external_single_structure(
                        array(
                            'id'              => new external_value(PARAM_INT,  'rating id'),
                            'userid'          => new external_value(PARAM_INT,  'user id'),
                            'userpictureurl'  => new external_value(PARAM_URL,  'URL user picture'),
                            'userfullname'    => new external_value(PARAM_NOTAGS, 'user fullname'),
                            'rating'          => new external_value(PARAM_NOTAGS, 'rating on scale'),
                            'timemodified'    => new external_value(PARAM_INT,  'time modified (timestamp)')
                        ), 'Rating'
                    ), 'list of ratings'
                ),
                'warnings'  => new external_warnings(),
            )
        );
    }