$signer = new \AdamStipak\Webpay\Signer(
  $privateKeyFilepath,    // Path of private key.
  $privateKeyPassword,    // Password for private key.
  $publicKeyFilepath      // Path of public key.
);
    
$api = new \AdamStipak\Webpay\Api(
  $merchantNumber,    // Merchant number.
  $webpayUrl,         // URL of webpay.
  $signer             // instance of \AdamStipak\Webpay\Signer.
);

use \AdamStipak\Webpay\PaymentRequest;

$request = new PaymentRequest(...);

$url = $api->createPaymentRequestUrl($request); // $api instance of \AdamStipak\Webpay\Api

// use $url as you want. In most cases for redirecting to GP Webpay.

// for minimal setup you can use
use \AdamStipak\Webpay\PaymentRequest;
use AdamStipak\Webpay\PaymentRequest\AddInfo;
 
$schema = file_get_contents("Path to XSD schema (GPwebpayAdditionalInfoRequest_v.4.xsd) from portal https://portal.gpwebpay.com/");
// use minimal valid values for XSD schema
$addInfo = new AddInfo($schema, AddInfo::createMinimalValues());
// or you can use valid values against XSD schema (start here AddInfo::createMinimalValues())
$request = new AddInfo($schema, [...]);

$request = new PaymentRequest(..., $addInfo);

use \AdamStipak\Webpay\PaymentResponse;
use \AdamStipak\Webpay\Exception;
 
$response = new PaymentResponse(...); // fill response with response parameters (from request).
 
try {
  $api->verifyPaymentResponse($response);
} 
catch (PaymentResponseException $e) {
  // PaymentResponseException has $prCode, $srCode for properties for logging GP Webpay response error codes.
}
catch (Exception $e) {
  // Digest is not correct.
}