code: public function setAsSubscribed(\DateTime $subscriptionDate, $subscriptionIp = '127.0.0.1')
      {
          $this->subscriptionStatus = self::STATUS_SUBSCRIBED;
          $this->subscriptionDate   = $subscriptionDate;
          $this->ipAddress          = $subscriptionIp;
  
          return $this;
      }
docs: Set this recipient as subscribed.
  @param \DateTime $subscriptionDate The recipient subscripiton date
  @param string    $subscriptionIp The recipient subscription IP
  
  @return Recipient