public function isValidSource(?string $id): bool
    {
        if ($id == '') {
            return false;
        }

        if ($this->saveHandler != null && method_exists($this->saveHandler, 'isValidSource')) {
            return $this->saveHandler->isValidSource($id);
        }

        // for 'sess_' @see https://github.com/php/php-src/blob/master/ext/session/mod_files.c#L85
        return file_exists($this->savePath .'/sess_'. $id);
    }