code: private function setAuth($username, $password)
      {
          if (strlen($username) > 255 || strlen($password) > 255) {
              throw new InvalidArgumentException('Both username and password MUST NOT exceed a length of 255 bytes each');
          }
          $this->auth = pack('C2', 0x01, strlen($username)) . $username . pack('C', strlen($password)) . $password;
      }
docs: set login data for username/password authentication method (RFC1929)
  
  @param string $username
  @param string $password
  @link http://tools.ietf.org/html/rfc1929