$gateway = \Omnipay\Omnipay::create('Sisow');
    $gateway->initialize(array(
        'shopId' => '',
        'merchantId' => '0123456',
        'merchantKey' => 'b36d8259346eaddb3c03236b37ad3a1d7a67cec6',
        'testMode' => true,
    ));
    // Start the purchase
    if(!isset($_GET['trxid'])){
        $url = "https://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
        $response = $gateway->purchase(array(
            'amount' => "6.84",
            'description' => "Testorder #1234",
            'issuer' => 99,                         // Get the id from the issuers list, 99 = test issuer
            //'paymentMethod' => 'overboeking',     // For 'overboeking', extra parameters are required:
            'card' => array(
                'email' => 'barry@fruitcakestudio.nl',
                'firstName' => 'Barry',
                'lastName' => 'vd. Heuvel',
                'company' => 'Fruitcake Studio',
            ),
            'transactionId' => 1234,
            'returnUrl' => $url,
            'notifyUrl' => $url,
        ))->send();
        if ($response->isRedirect()) {
            // redirect to offsite payment gateway
            $response->redirect();
        } elseif ($response->isPending()) {
            // Process started (for example, 'overboeking')
            return "Pending, Reference: ". $response->getTransactionReference();
        } else {
            // payment failed: display message to customer
            return "Error " .$response->getCode() . ': ' . $response->getMessage();
        }
    }else{
        // Check the status
        $response = $gateway->completePurchase()->send();
        if($response->isSuccessful()){
            $reference = $response->getTransactionReference();  // TODO; Check the reference/id with your database
            return "Transaction '" . $response->getTransactionId() . "' succeeded!";
        }else{
            return "Error " .$response->getCode() . ': ' . $response->getMessage();
        }
    }

$response = Omnipay::fetchIssuers()->send();
if($response->isSuccessful()){
    print_r($response->getIssuers());
}