CartItem.php

<?php

class CartItem {
    private Product $product;
    private int $quantity;

    /**
     * CartItem constructor.
     *
     * @param \Product $product
     * @param int      $quantity
     */
    public function __construct (\Product $product, $quantity) {
        $this->product = $product;
        $this->quantity = $quantity;
    }

    /**
     * @return \Product
     */
    public function getProduct() {
        return $this->product;
    }

    /**
     * @param \Product $product
     */
    public function setProduct($product) {
        $this->product = $product;
    }

    /**
     * @return int
     */
    public function getQuantity() {
        return $this->quantity;
    }

    /**
     * @param int $quantity
     */
    public function setQuantity($quantity){
        $this->quantity = $quantity;
    }

    public function increaseQuantity($amount = 1) {
        if($this->getQuantity() + $amount > $this->getProduct()->getAvailableQuantity()) {
            throw new Exception("Product quantity can not be more than ".$this->getProduct()->getAvailableQuantity());
        }
        $this->quantity += $amount;
    }

    public function decreaseQuantity($amount = 1) {
        if ($this->getQuantity() - $amount < 1) {
            throw new Exception("Product quantity can not be less than 1");
        }
        $this->quantity -= $amount;
    }

}