gistfile1.txt

class cart {
    public $item;
    
    public function getItem() {
        return $this->item;
    }

    public function setItem(&$item) {
        $this->item = $item;
    }

    public function addItem($id, $quantity = 1){
            $array = array(
                'id' => $id,
                'quantity' => $quantity
            );
            if($this->checkStock($id, $array['quantity'])){ 
                $this->item[] = $array;
                var_dump($this->item);
                return true;
            }

        return false;
    }

    public function getItems(){
        var_dump($this->item);
        return $this->item;
    }

}

gistfile2.txt

            $cart = new cart();
            if(!isset($_SESSION['cart'])){
                $_SESSION['cart'] = array();
            }
            $cart->setItem($_SESSION['cart']);

hi i have corrected the error it was in setItem function you can find it here https://gist.github.com/usm4n/7049339