public static function compare(string $left, string $right, int $len = null): bool
    {
        if (\class_exists('\Normalizer', false)) {
            $left = \Normalizer::normalize($left, \Normalizer::FORM_D); // form NFD is faster
            $right = \Normalizer::normalize($right, \Normalizer::FORM_D); // form NFD is faster
        }

        if ($len < 0) {
            $left = \mb_substr($left, $len, -$len);
            $right = \mb_substr($right, $len, -$len);
        } elseif ($len !== null) {
            $left = \mb_substr($left, 0, $len);
            $right = \mb_substr($right, 0, $len);
        }

        return \mb_strtolower($left) === \mb_strtolower($right);
    }