You can make simple check of merchant order with isMerchantOrderPaid() method. It checks payment status is success. Also, it checks amount and currency of merchant order match Paysera payment amount and currecy.
<?php
use Paysera\CheckoutSdk\CheckoutFacade;
use Paysera\CheckoutSdk\CheckoutFacadeFactory;
use Paysera\CheckoutSdk\Entity\Request\PaymentCallbackValidationRequest;
use Paysera\CheckoutSdk\Entity\Order;
...
$merchantOrder = new Order(
(int) $data['order_id'],
(int) $data['order_amount_in_cents'],
(string) $order['order_currency_code']
);
$checkoutFacade = (new CheckoutFacadeFactory)->create();
$paymentValidationRequest = new PaymentCallbackValidationRequest(
(int) $data['project_id'],
(string) $data['project_password'],
(string) $data['payment_request_data']
);
$paymentValidationRequest->setSs1($data['payment_request_ss1'])
->setSs2($data['payment_request_ss2'])
->setSs3($data['payment_request_ss3']);
$response = $checkoutFacade->getPaymentCallbackValidatedData($paymentValidationRequest);
$isPaid = $checkoutFacade->isMerchantOrderPaid($response, $merchantOrder);
if ($isPaid) {
// Do something (update order or etc.)
}