array_key_exists() expects parameter 2 to be array, null given - Cakephp 3

Friends, I appreciate any comments.

I'm having problems with array_key_exists. I need to check if a product already exists in the shopping cart. I'm doing it this way:

.
$session = $this->request->getSession();
            $cart = $session->read( 'cart' );
            
            if (array_key_exists($order->product_id, (array) $cart)) {
                $this->Flash->error('Invalid request');

            } else {
                $cart[] = $order;
                $session->write('cart', $cart );
                $this->Flash->success($order->product->name_product . ' has been added to the shopping cart');
                return $this->redirect( ['action' => 'index'] );

            }
            return $this->redirect($this->referer());
        } else {
            return $this->redirect(['action' => 'index']);
        }
.
.

I received the error below, but now with the update it no longer exists. But the function still doesn't help me.

array_key_exists() expects parameter 2 to be array, int given

The product is added to the cart even if it has already been added. I need to compare the selected product id with the product id that is already in the cart. The order information is basically: id, quantity, user_id, product_id.

I am grateful if anyone can analyze some way to check the id in the array that is mounted.

When you do this:

$cart[] = $order;

you are asking PHP to add a new element to the end of your $cart array, giving it the next available numeric key. You are then trying to see if the product id exists as a key in the array. Unless your product ids are like 0 and 1, that's unlikely to ever happen. If you want your cart to be keyed by product id, use

$cart[$order->product_id] = $order;