isNullType example

        $encoded = $value;

        if (mb_detect_encoding($value, 'auto', true) !== 'UTF-8') {
            $encoded = mb_convert_encoding($value, 'UTF-8', 'Windows-1252');
        }

        if ($this->isValid($encoded)) {
            return $this->charValue[$encoded];
        }

        if ($this->isNullType($encoded)) {
            return self::C_NUL;
        }

        if ($this->isInvalidChar($encoded)) {
            $this->hasInvalidTokens = true;
            return self::INVALID;
        }

        return self::GENERIC;
    }