Slim PHP - POST array to Render Template View

Question

Using SLIM PHP with Controllers.

Looking to POST an product array to the checkout View.

My Routes

$app->get('/shop/product/:slug/', array(new \Controller\ShopController(), "getProduct"));
$app->post('/shop/product/:slug/', array(new \Controller\ShopController(), "addToCheckout"));
$app->get('/shop/checkout/', array(new \Controller\CheckoutController(), "index")); 

POST addToCheckout() function

public function addToCheckout() {

    $req = $this->app->request()->post();

    $productId = $req['productId'];
    $productSize = $req['size'];
    $productQuantity = $req['quantity'];

    // pass array to /shop/checkout/ View
    $product = array(
         'id' => $productId,
         'size' => $productSize,
         'quantity' => $productQuantity
    );

    $this->app->response->redirect('/shop/checkout/');

}

GET checkout/index function (looking to pass the product array through to here)

public function index() {

        $title = "Checkout | Shop";

        $this->render('shop/checkout/index.html', array("products" => $products, "title" => $title));

    }