Mage2\Checkout\Controllers\CheckoutController::postPaymentOption PHP Method

    public function postPaymentOption(PaymentOptionRequest $request)
    {
        $orderData = Session::get('order_data');
        $cartProducts = Session::get('cart');
        $paymentMethod = Payment::get($request->get('payment_option'));
        $redirectUrl = $paymentMethod->process($orderData, $cartProducts);
        $orderData['payment_method'] = $request->get('payment_option');
        Session::put('order_data', $orderData);
        if (null === $redirectUrl) {
            return redirect()->route('checkout.step.review');
        } else {
            return Redirect::to($redirectUrl);
        }
    }