code: public function send($msgType, $message, $to = null)
      {
          $message = (new MessageBuilder())->msgType($msgType)->message($message)->to($to)->build();
  
          $api = is_array($to) ? self::API_SEND_BY_OPENID : self::API_SEND_BY_GROUP;
  
          return $this->post($api, $message);
      }
docs: Send a message.
  
  @param string $msgType message type
  @param mixed  $message message
  @param mixed  $to
  
  @return mixed