WitShop/app/Services/CartService.php

<?php

namespace App\Services;

use App\Models\ProductVariant;
use Illuminate\Support\Facades\Session;

class CartService
{
    private const SESSION_KEY = 'cart';

    public function get(): array
    {
        return Session::get(self::SESSION_KEY, []);
    }

    public function add(int $variantId, int $qty = 1): void
    {
        $cart = $this->get();
        $key = "v{$variantId}";
        if (isset($cart[$key])) {
            $cart[$key]['qty'] += $qty;
        } else {
            $variant = ProductVariant::with('product')->findOrFail($variantId);
            $cart[$key] = [
                'variant_id'   => $variantId,
                'product_name' => $variant->product->name,
                'variant_label'=> $variant->label,
                'price'        => $variant->effective_price,
                'tax_rate'     => $variant->product->tax_rate,
                'qty'          => $qty,
            ];
        }
        Session::put(self::SESSION_KEY, $cart);
    }

    public function update(string $key, int $qty): void
    {
        $cart = $this->get();
        if (isset($cart[$key])) {
            if ($qty <= 0) {
                unset($cart[$key]);
            } else {
                $cart[$key]['qty'] = $qty;
            }
        }
        Session::put(self::SESSION_KEY, $cart);
    }

    public function remove(string $key): void
    {
        $cart = $this->get();
        unset($cart[$key]);
        Session::put(self::SESSION_KEY, $cart);
    }

    public function clear(): void
    {
        Session::forget(self::SESSION_KEY);
    }

    public function subtotal(): float
    {
        return array_sum(array_map(fn($i) => $i['price'] * $i['qty'], $this->get()));
    }

    public function count(): int
    {
        return array_sum(array_column($this->get(), 'qty'));
    }
}