protected function buildSearchDirs(/*# string */ $env)/*# : array */
    {
        $path = $this->root_dir;
        $part = preg_split(
            '/[\/\\\]/',
            trim($env, '/\\'),
            0,
            \PREG_SPLIT_NO_EMPTY
        );
        $subdirs = [$path];
        foreach ($part as $dir) {
            $path .= $dir . \DIRECTORY_SEPARATOR;
            if (false === file_exists($path)) {
                trigger_error(
                    Message::get(Message::CONFIG_ENV_UNKNOWN, $env),
                    \E_USER_WARNING
                );
                break;
            }
            $subdirs[] = $path;
        }
        return $subdirs;
    }