UpdateStock.php

<?php

namespace App\Listeners;

use App\Events\OrderCreated;
use App\Events\StockIsLow;
use App\Product;

class UpdateStock
{
    /**
     * Create the event listener.
     *
     * @return void
     */
    public function __construct()
    {
        //
    }

    /**
     * Handle the event.
     *
     * @param  OrderCreated  $event
     * @return void
     */
    public function handle(OrderCreated $event)
    {
        foreach ($event->order->items as $item) {
            $product = Product::find($item->productId);
            
            $quantityLeft = $product->quantity - $item->quantity;
            
            $product->update([
                'quantity' => $quantityLeft
            ]);
            
            if ($quantityLeft < 3) {
                event(new StockIsLow($product));
            }
        }
    }
}