Sonata\Component\Payment\TransactionInterface::isValid PHP Method

    public function isValid();

 /**
  * {@inheritdoc}
  */
 public function sendConfirmationReceipt(TransactionInterface $transaction)
 {
     if (!$transaction->isValid()) {
         return new \Symfony\Component\HttpFoundation\Response('');
     }
     $params = $transaction->getParameters();
     $params['cmd'] = '_notify-validate';
     //$this->getLogger()->
     // retrieve the client
     $client = $this->getWebConnectorProvider()->getNamedClient($this->getOption('web_connector_name', 'default'));
     $client->request('POST', $this->getOption('url_action'), $params);
     if ($client->getResponse()->getContent() == 'VERIFIED') {
         $transaction->setState(TransactionInterface::STATE_OK);
         $transaction->setStatusCode(TransactionInterface::STATUS_VALIDATED);
         $transaction->getOrder()->setValidatedAt(new \DateTime());
         $transaction->getOrder()->setStatus(OrderInterface::STATUS_VALIDATED);
         $transaction->getOrder()->setPaymentStatus(TransactionInterface::STATUS_VALIDATED);
     } else {
         $transaction->setState(TransactionInterface::STATE_KO);
         $transaction->setStatusCode(TransactionInterface::STATUS_ERROR_VALIDATION);
         // TODO error in status -> setting payment status to an order status value
         $transaction->getOrder()->setPaymentStatus(OrderInterface::STATUS_ERROR);
         if ($this->getLogger()) {
             $this->getLogger()->emergency('[Paypal::sendAccuseReception] Paypal failed to check the postback');
         }
     }
     return new \Symfony\Component\HttpFoundation\Response('');
 }