protected static function getFiles($file)
    {
        if (empty($file)) {
            throw new InvalidArgumentException('There is not any file defined');
        }

        if (is_string($file)) {
            if (!is_file($file)) {
                throw new InvalidArgumentException("'$file' is not a valid file");
            }

            if (!is_readable($file)) {
                throw new InvalidArgumentException("'$file' is not a readable file");
            }

            return [$file];
        }

        if (is_array($file)) {
            $files = [];

            foreach ($file as $f) {
                $files = array_merge($files, self::getFiles($f));
            }

            return $files;
        }

        throw new InvalidArgumentException('The first argument must be string or array');
    }