src/Payum/Paypal/Ipn/Api.php
public function notifyValidate(array $fields)
    {
        $fields['cmd'] = self::CMD_NOTIFY_VALIDATE;

        $headers = array(
            'Content-Type' => 'application/x-www-form-urlencoded',
        );

        $request = $this->messageFactory->createRequest('POST', $this->getIpnEndpoint(), $headers, http_build_query($fields));

        $response = $this->client->send($request);

        if (false == ($response->getStatusCode() >= 200 && $response->getStatusCode() < 300)) {
            throw HttpException::factory($request, $response);
        }

        $result = $response->getBody()->getContents();

        return self::NOTIFY_VERIFIED === $result ? self::NOTIFY_VERIFIED : self::NOTIFY_INVALID;
    }
public function notifyValidate(array $fields)
    {
        $fields['cmd'] = self::CMD_NOTIFY_VALIDATE;

        $headers = array(
            'Content-Type' => 'application/x-www-form-urlencoded',
        );

        $request = $this->messageFactory->createRequest('POST', $this->getIpnEndpoint(), $headers, http_build_query($fields));

        $response = $this->client->send($request);

        if (false == ($response->getStatusCode() >= 200 && $response->getStatusCode() < 300)) {
            throw HttpException::factory($request, $response);
        }

        $result = $response->getBody()->getContents();

        return self::NOTIFY_VERIFIED === $result ? self::NOTIFY_VERIFIED : self::NOTIFY_INVALID;
    }
@param array $fields

@return string