protected static function parseValidKeys()
    {
        if (self::$validOptions !== null) {
            return;
        }

        $refl = new ReflectionClass('\ZMQ');
        $const = $refl->getConstants();
        self::$validOptions = array();
        foreach ($const as $key => $value) {
            if (substr($key, 0, 8) == 'SOCKOPT_') {
                self::$validOptions[$value] = '\ZMQ::'.$key;
            }
        }
    }