// minimal configuration
$gopay = GoPay\Api::payments([
    'goid' => 'my goid',
    'clientId' => 'my id',
    'clientSecret' => 'my secret',
    'gatewayUrl' => 'gateway url'
]);

// full configuration
$gopay = GoPay\Api::payments([
    'goid' => 'my goid',
    'clientId' => 'my id',
    'clientSecret' => 'my secret',
    'gatewayUrl' => 'gateway url',
    'scope' => GoPay\Definition\TokenScope::ALL,
    'language' => GoPay\Definition\Language::CZECH,
    'timeout' => 30
]);

$response = $gopay->createPayment([/* define your payment  */]);
if ($response->hasSucceed()) {
    echo "hooray, API returned {$response}";
    return $response->json['gw_url']; // url for initiation of gateway
} else {
    // errors format: https://doc.gopay.com/en/?shell#http-result-codes
    echo "oops, API returned {$response->statusCode}: {$response}";
}

// create payment and pass url to template 
$response = $gopay->createPayment([/* define your payment  */]);
if ($response->hasSucceed()) {
    $templateParameters = [
        'gatewayUrl' => $response->json['gw_url'],
        'embedJs' => $gopay->urlToEmbedJs()
    ];
    // render template
}

<form action="<?php echo $gatewayUrl; ?>" method="post" id="gopay-payment-button">
  <button name="pay" type="submit">Pay</button>
  <script type="text/javascript" src="<?php echo $embedJs;>"></script>
</form>

<form action="<?php echo $gatewayUrl; ?>" method="post">
  <button name="pay" type="submit">Pay</button>
</form>

// register cache in optional service configuration
$gopay = GoPay\payments(
    [/* your config */],
    ['cache' => new PrimitiveFileCache()]
);

<?php

use GoPay\Token\TokenCache;
use GoPay\Token\AccessToken;

class PrimitiveFileCache implements TokenCache
{
    public function setAccessToken($client, AccessToken $t)
    {
        file_put_contents(__DIR__ . "/{$client}", serialize($t));
    }

    public function getAccessToken($client)
    {
        $file = __DIR__ . "/{$client}";
        if (file_exists($file)) {
            return unserialize(file_get_contents($file));
        }
        return null; 
    }
}

// register logger in optional service configuration
$gopay = GoPay\payments(
    [/* your config */],
    ['logger' => new GoPay\Http\Log\PrintHttpRequest()]
);