A shared room, a copied box

1<?php2class Cart3{4    private array $items = ['book'];5 6    public function items(): array7    {8        return $this->items;9    }10 11    public function add(string $item): void12    {13        $this->items[] = $item;14    }15}16$cart = new Cart();17$list = $cart->items();18$list[] = 'mug';19echo count($cart->items()), PHP_EOL;20$cart->add('pen');21echo implode(',', $cart->items()), PHP_EOL;22echo implode(',', $list), PHP_EOL;