haphpiness

Readonly Properties and Classes

PHP 8.1 added readonly properties — set once, then immutable. PHP 8.2 extended this to entire classes. Immutability is no longer a convention; it's enforced by the engine.

// Readonly properties (PHP 8.1)
class Invoice {
    public function __construct(
        public readonly string $number,
        public readonly float $total,
        public readonly DateTimeImmutable $issuedAt,
    ) {}
}

$invoice = new Invoice('INV-001', 250.00, new DateTimeImmutable());
$invoice->total = 0; // Error: Cannot modify readonly property Invoice::$total

// Readonly classes (PHP 8.2) — all properties are implicitly readonly
readonly class Money {
    public function __construct(
        public int $amount,
        public string $currency,
    ) {}

    public function add(Money $other): self {
        if ($this->currency !== $other->currency) {
            throw new \InvalidArgumentException('Currency mismatch');
        }
        return new self($this->amount + $other->amount, $this->currency);
    }
}

$price = new Money(1000, 'EUR');
$tax = new Money(210, 'EUR');
$total = $price->add($tax); // New object — originals unchanged

Readonly properties make value objects trivial to implement correctly. No more writing private properties with getters, no more worrying about someone mutating shared state. The language guarantees immutability.