What is immutability and value objects in PHP?

Immutable objects cannot be changed after creation — every "modification" returns a new object with the changed state. In PHP, implement with readonly properties (PHP 8.1) or by returning new instances from methods: public function withCurrency(string $currency): static { return new static($this->amount, $currency); }. Value Objects are immutable objects defined entirely by their values (not an identity/ID) — two Money objects with the same amount and currency are equal. Common value objects: Money, Email, Address, DateRange, Color. Benefits: thread safety, predictable behavior (no side effects), easy to test, and can be safely shared. PHP 8.1 readonly classes make immutable value objects trivial: readonly class Money { public function __construct(public int $amount, public string $currency) {} }.