code: public function setPaymentType($paymentType)
      {
          if (self::PAYMENT_PEER_TO_PEER !== $paymentType) {
              throw new RuntimeException(Tools::poorManTranslate('fts-shared', 'Invalid payment type.'));
          }
  
          return $this->add('PT', $paymentType);
      }
docs: Set payment type
  
  @param string $paymentType Payment type - see constants PAYMENT_*
  @return QRPayment
  @throws RuntimeException