Guarding aggregate with invariant

Order.php

<?php

class Order
{
    protected $basket = [];

    protected $maxSize;
    protected $confirmed = false;
    private function __construct($product)
    {
        $this->product = $product;
        $this->maxSize = 10;
    }
    public static function instance($product)
    {
        return new static($product);
    }
    public function add($product)
    {
        if (count($this->basket) >= $this->maxSize) {
            throw new \Exception('Invariant broken!!!');
        }
        $this->products[] = $product;
    }
    public function confirm()
    {
        if ($this->confirmed == true) {
            throw new \Exception('Order is already confirmed!!!');
        }
        $this->confirmed = true;
    }
}
$order = Order::instance(10);
for ($i= 0; $i < 2; $i++) {
    $order->add(10);
}
$order->confirm();
$order->confirm();