Payment Systems Integration in OpenCart

A payment system is a critically important component of an online store. In this article, we'll look at integrating popular payment providers into OpenCart.

General Payment Module Architecture

Each payment module in OpenCart 4 consists of:

• Admin Controller — module settings
• Catalog Controller — payment processing
• Model — database interaction
• Twig templates — payment form

LiqPay Integration

LiqPay is a popular payment system in Ukraine:

<?php
// catalog/controller/extension/payment/liqpay.php
namespace Opencart\Catalog\Controller\Extension\Payment;

class Liqpay extends \Opencart\System\Engine\Controller {

    public function index(): string {
        $this->load->language('extension/payment/liqpay');

        $order_id = $this->session->data['order_id'];
        $this->load->model('checkout/order');
        $order = $this->model_checkout_order->getOrder($order_id);

        $data = base64_encode(json_encode([
            'version' => 3,
            'public_key' => $this->config->get('payment_liqpay_public_key'),
            'action' => 'pay',
            'amount' => $order['total'],
            'currency' => $order['currency_code'],
            'description' => 'Order #' . $order_id,
            'order_id' => $order_id,
            'result_url' => $this->url->link('checkout/success'),
            'server_url' => $this->url->link('extension/payment/liqpay|callback')
        ]));

        $signature = base64_encode(sha1(
            $this->config->get('payment_liqpay_private_key') .
            $data .
            $this->config->get('payment_liqpay_private_key'),
            true
        ));

        return $this->load->view('extension/payment/liqpay', [
            'data' => $data,
            'signature' => $signature
        ]);
    }

    public function callback(): void {
        $data = json_decode(base64_decode($_POST['data']), true);

        // Signature verification
        $signature = base64_encode(sha1(
            $this->config->get('payment_liqpay_private_key') .
            $_POST['data'] .
            $this->config->get('payment_liqpay_private_key'),
            true
        ));

        if ($signature !== $_POST['signature']) {
            die('Invalid signature');
        }

        if ($data['status'] === 'success') {
            $this->load->model('checkout/order');
            $this->model_checkout_order->addHistory(
                $data['order_id'],
                $this->config->get('payment_liqpay_order_status_id'),
                'LiqPay payment successful'
            );
        }
    }
}

Stripe Integration

// Using Stripe PHP SDK
require_once DIR_SYSTEM . 'library/stripe/autoload.php';

\Stripe\Stripe::setApiKey($this->config->get('payment_stripe_secret_key'));

$paymentIntent = \Stripe\PaymentIntent::create([
    'amount' => $order['total'] * 100, // in cents
    'currency' => strtolower($order['currency_code']),
    'metadata' => [
        'order_id' => $order_id
    ]
]);

// Webhook handling
$payload = file_get_contents('php://input');
$sig_header = $_SERVER['HTTP_STRIPE_SIGNATURE'];

$event = \Stripe\Webhook::constructEvent(
    $payload,
    $sig_header,
    $this->config->get('payment_stripe_webhook_secret')
);

if ($event->type === 'payment_intent.succeeded') {
    $order_id = $event->data->object->metadata->order_id;
    // Update order status
}

Use webhooks for payment confirmation instead of redirect. The client may close the browser before redirect.

— Best Practice

PayPal Checkout

// Frontend: PayPal JavaScript SDK
paypal.Buttons({
    createOrder: function(data, actions) {
        return fetch('/index.php?route=extension/payment/paypal|createOrder', {
            method: 'POST',
            headers: {'Content-Type': 'application/json'},
            body: JSON.stringify({order_id: orderId})
        })
        .then(res => res.json())
        .then(data => data.paypal_order_id);
    },
    onApprove: function(data, actions) {
        return fetch('/index.php?route=extension/payment/paypal|captureOrder', {
            method: 'POST',
            headers: {'Content-Type': 'application/json'},
            body: JSON.stringify({paypal_order_id: data.orderID})
        })
        .then(res => res.json())
        .then(data => {
            if (data.success) {
                window.location.href = '/checkout/success';
            }
        });
    }
}).render('#paypal-button-container');

Testing Payments

• LiqPay: sandbox mode in settings
• Stripe: test keys + card 4242424242424242
• PayPal: sandbox accounts

Logging and Monitoring

// Payment logging
$this->log->write('Payment: Order #' . $order_id . ' - Status: ' . $status);

// Log structure
[
    'timestamp' => date('Y-m-d H:i:s'),
    'order_id' => $order_id,
    'payment_method' => 'liqpay',
    'amount' => $amount,
    'status' => $status,
    'transaction_id' => $transaction_id
]

Conclusion

Proper payment system integration ensures security and convenience for customers. Don't forget about webhook confirmation, logging, and testing all scenarios.