readonly Properties (PHP 8.1)

TL;DR

Explanation

public readonly string $name; cannot be written after initialization. Writing a second time throws Error. Cannot be unset. Must be typed. Cannot have a default value in declaration (except via constructor promotion). Combined with constructor promotion: public function __construct(public readonly string $name) — declare + promote + make readonly in one line. PHP 8.2 readonly classes: mark the entire class and all promoted properties are readonly. PHP 8.3 allows readonly property modification in __clone() for clone-with patterns. Use readonly for value objects, DTOs, request objects.

Common Misconception

Why It Matters

Common Mistakes

• Trying to set readonly property after construction — throws Error.
• Forgetting readonly requires a type declaration.
• Not using readonly class (8.2) when all properties should be immutable.

Code Examples

class Money {
    private int $amount;
    private string $currency;

    public function getAmount(): int { return $this->amount; }
    public function getCurrency(): string { return $this->currency; }
}

// PHP 8.1 — readonly + constructor promotion:
class Money {
    public function __construct(
        public readonly int $amount,
        public readonly string $currency,
    ) {}
}

// PHP 8.2 — readonly class:
readonly class Money {
    public function __construct(
        public int $amount,
        public string $currency,
    ) {}
}

References

• ↗ https://www.php.net/manual/en/language.oop5.properties.php#language.oop5.properties.readonly-properties

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