class Order
{
    private Product $product;
    private int $quantity;
    public function __construct(Product $product, int $quantity)
    {
        if ($quantity <= 0) {
            throw new MinQuantityException();
        }
    }
}

$order->getStatus();
// isn't delivered
$order->setCancel();

class Order
{
    // ...
    public function cancel(): void
    {
        if ($this->status === STATUS::DELIVERED) {
            throw new LogicException(
                sprintf(
                    'Order %s has already been delivered',
                    $this->id->asString()
                )
            );
        }
        $this->status = STATUS::CANCEL;
    }
}

class Account
{
    private string $accountNumber;
    private float $amount = 0.00;
    private string $currency = 'USD';
    const NUMBER_OF_CHARACTERS = 16;
    public function __construct(string $accountNumber)
    {
        if (strlen($accountNumber) !== self::NUMBER_OF_CHARACTERS) {
            // throw exception
        }
        $this->accountNumber = $accountNumber;
    }
    public function putMoney(float $amount, string $currency)
    {
        if ($amount <= 0) {
            // throw exception
        }
        if ($currency !== $this->currency) {
            //thow exception
        }
        $this->amount += $amount;
    }
}

class AccountNumer
{
    const NUMBER_OF_CHARACTERS = 16;
    private string $accountNumber;
    public function __construct(string $accountNumber)
    {
        if (strlen($accountNumber) !== self::NUMBER_OF_CHARACTERS) {
            // throw exception
        }
        // another rules
        $this->accountNumber = $accountNumber;
    }
    public function __toString(): string
    {
        return $this->accountNumber;
    }
}

class Currency extends SplEnum // Пока не 8.1
{
    const __default = self::USD;
    const USD = 'USD';
    const EUR = 'EUR';
    // etc.
}
class Money
{
    private float $amount;
    private string $currency;
    public function __construct(float $amount, Currency $currency)
    {
        if ($amount <= 0) {
            // throw exception
        }
        $this->amount = $amount;
        $this->currency = $currency;
    }
    public function currency(): string
    {
        return $this->currency;
    }
    public function amount(): float
    {
        return $this->amount;
    }
    public function append(Money $money)
    {
        if ($money->currency() !== $this->currency) {
            // throw exception
        }
        $amount = $this->amount + $money->amount();
        return new self($amount, $this->currency);
    }
}

class Account
{
    private AccountNumer $accountNumber;
    private Money $money;
    public function __construct(AccountNumer $accountNumber)
    {
        $this->accountNumber = $accountNumber;
        $this->money = new Money(0.00, 'USD');
    }
    public function putMoney(Money $money)
    {
        $this->money = $this->money->append($money);
    }
}