Laravel 8 Shopping Cart item quantity wont increment

I'm a beginner in laravel and I've been trying to create a simple ecommerce web but i got stuck in this logic error for days. I've been following in a youtube tutorial about creating shopping cart but the item quantity won't increment if i add the same item in the cart. The totalPrice and totalQty seems counting fine though...
this is what i get when i tried to dd($this) into cart.php.

I think it's because i declared the array in the add function(in Cart.php) and it keeps overwriting the current quantity. but i cant declare it outside of the function or it'll throws an error saying undefined index. I've tried checking some of relevant posts here but it still didn't work. I don't know what to do anymore.
Do you guys know what's the solution? Thank you!

p/s: this is the video tutorial link that i watched. all creds to Academind for an amazing tutorial.

https://www.youtube.com/watch?v=4J939dDUH4M&list=PL55RiY5tL51qUXDyBqx0mKVOhLNFwwxvH&index=9

these are my related codes to the error.

In Cart.php

<?php

namespace App\Models;

class Cart
{
    public $items = array();
    public $totalQty = 0;
    public $totalPrice = 0;


    public function __construct($oldCart)
    {
        if ($oldCart) {
            //dd($this);
            $this->items = $oldCart->items;
            $this->totalQty = $oldCart->totalQty;
            $this->totalPrice = $oldCart->totalPrice;
        }
    }


    public function add($item, $id)
    {
        //force get $items
        global $items;

        //default values if item not in cart
        $storedItem = [
            'qty' => 0,
            'price' => $item->price,
            'item' => $item
        ];
        
        //if item is already in shopping cart
        if (isset($this->$items) ?? $items::false) {
            if (array_key_exists($id, $this->items)) {
                $storedItem = $this->items[$id];
            }
        }


        $storedItem['qty']++;
        $storedItem['price'] = $item->price * $storedItem['qty'];
        $this->items[$id] = $storedItem;
        $this->totalQty++;
        $this->totalPrice += $item->price;
        dd($this);
    }
}

In FoodController.php

 <?php

namespace App\Http\Controllers;

use App\Models\Cart;
use App\Models\Food;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Session;

public function addtoCart(Request $request, $id, Food $foods)
        {
    
            $foods = Food::find($id);
            //check in session if cart already contains product
            $oldCart = Session::has('cart') ? Session::get('cart') : null;

            //if it did contain products, pass them to constructor
            $cart = new Cart($oldCart);
    
            $cart->add($foods, $foods->id);
    
            $request->session()->put('cart', $cart);
    
    
            return view('cart', ['food' => $cart->items, 'totalPrice' => $cart->totalPrice]);
        }
    
    public function getCart()
    {
        if (!Session::has('cart')) {
            return view('cart', ['food' => null]);
        }
    
        $oldCart = Session::get('cart');
        $cart = new Cart($oldCart);
        return view('cart', ['food' => $cart->items,'totalPrice' =>$cart->totalPrice]);
        }

in cart.blade.php

@foreach ($food as $foods)
                    
                    
    <p align="left"><a href="#">  </a> <span class = "price"> $ </span></p>
    <p align="left">Quantity</p>
    <p><input class="w3-input w3-p-adding-16 w3-border" type="number" placeholder="Quantity" value="" required name="quantity"></p>
                   
@endforeach
                    
                    
<hr class="new1">
<p align="left">Total <span class="price"><b> $ </b></span></p>

via Chebli Mohamed