private function getMbCharset($cs)
    {
        $normalized = $this->getNormalizedCharset($cs);
        if (array_key_exists($normalized, self::$mbListedEncodings)) {
            return self::$mbListedEncodings[$normalized];
        } elseif (array_key_exists($normalized, self::$mbAliases)) {
            return self::$mbAliases[$normalized];
        }
        return false;
    }