Payum Paypal on Symfony3 - symfony

Question

I'm trying to integrate a payment system of paypal on my webpage under Symfony. After some researches, I ran into Payum which is a apparently the best bundle for this feature.
The issue is that I don't really understand the doc so the final code I have doesn't work.
Somebody already used payum and could help me to understand it ?
I have for example : $paypalRestPaymentDetailsClass which comes from nowhere and I don't know what should be in this class
Here is my code :
namespace PlatformBundle\Controller;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Payum\Core\PayumBuilder;
use Payum\Core\Payum;
use PayPal\Api\Amount;
use PayPal\Api\Payer;
use PayPal\Api\RedirectUrls;
use PayPal\Api\Transaction;
use Payum\Core\Request\GetHumanStatus;
class PayumController extends Controller
{
private function config()
{
return (new PayumBuilder())
->addDefaultStorages()
->addGateway('gatewayName', [
'factory' => 'paypal_rest',
'client_id' => 'REPLACE IT',
'client_secret' => 'REPLACE IT',
'config_path' => 'REPLACE IT',
])
->getPayum();
}
public function prepare()
{
$payum = $this->config();
$storage = $payum->getStorage($paypalRestPaymentDetailsClass);
$payment = $storage->create();
$storage->update($payment);
$payer = new Payer();
$payer->payment_method = "paypal";
$amount = new Amount();
$amount->currency = "USD";
$amount->total = "1.00";
$transaction = new Transaction();
$transaction->amount = $amount;
$transaction->description = "This is the payment description.";
$captureToken = $payum->getTokenFactory()->createCaptureToken('paypalRest', $payment, 'create_recurring_payment.php');
$redirectUrls = new RedirectUrls();
$redirectUrls->return_url = $captureToken->getTargetUrl();
$redirectUrls->cancel_url = $captureToken->getTargetUrl();
$payment->intent = "sale";
$payment->payer = $payer;
$payment->redirect_urls = $redirectUrls;
$payment->transactions = array($transaction);
$storage->update($payment);
header("Location: ".$captureToken->getTargetUrl());
}
public function capture()
{
$payum = $this->config();
$token = $payum->getHttpRequestVerifier()->verify($_REQUEST);
$gateway = $payum->getGateway($token->getGatewayName());
if ($reply = $gateway->execute(new Capture($token), true)) {
if ($reply instanceof HttpRedirect) {
header("Location: ".$reply->getUrl());
die();
}
throw new \LogicException('Unsupported reply', null, $reply);
}
$payum->getHttpRequestVerifier()->invalidate($token);
header("Location: ".$token->getAfterUrl());
}
public function done()
{
$payum = $this->config();
$token = $payum->getHttpRequestVerifier()->verify($_REQUEST);
$gateway = $payum->getGateway($token->getGatewayName());
// you can invalidate the token. The url could not be requested any more.
// $payum->getHttpRequestVerifier()->invalidate($token);
// Once you have token you can get the model from the storage directly.
//$identity = $token->getDetails();
//$payment = $payum->getStorage($identity->getClass())->find($identity);
// or Payum can fetch the model for you while executing a request (Preferred).
$gateway->execute($status = new GetHumanStatus($token));
$payment = $status->getFirstModel();
header('Content-Type: application/json');
echo json_encode(array(
'status' => $status->getValue(),
'order' => array(
'total_amount' => $payment->getTotalAmount(),
'currency_code' => $payment->getCurrencyCode(),
'details' => $payment->getDetails(),
),
));
}
}
Thanks