protected function utf8Encode($s)
    {
        if (preg_match('//u', $s)) {
            return $s;
        }

        if (!\function_exists('iconv')) {
            throw new \RuntimeException('Unable to convert a non-UTF-8 string to UTF-8: required function iconv() does not exist. You should install ext-iconv or symfony/polyfill-iconv.');
        }

        if (false !== $c = @iconv($this->charset, 'UTF-8', $s)) {
            return $c;
        }
        if ('CP1252' !== $this->charset && false !== $c = @iconv('CP1252', 'UTF-8', $s)) {
            return $c;
        }

        return iconv('CP850', 'UTF-8', $s);
    }