Abstract readonly Properties

TL;DR

Explanation

PHP 8.4 introduced abstract readonly properties. An abstract class or interface can declare abstract public readonly string $name; requiring any concrete implementation to provide a readonly property with that name and type. This is more expressive than an abstract getter method — it declares structural intent. Implementations must declare the property as readonly (not just any property). This enables value-object hierarchies and interface-driven readonly DTOs. Before PHP 8.4, this required abstract getter methods as a workaround.

Common Misconception

Why It Matters

Common Mistakes

• Trying to use abstract readonly in PHP < 8.4 — syntax error.
• Implementing with a non-readonly property — type error.
• Confusing with readonly classes (PHP 8.2) which make all properties readonly.

Code Examples

// PHP 8.3 workaround — method instead of property:
abstract class Entity {
    abstract public function getId(): int;
}

// PHP 8.4:
abstract class Entity {
    abstract public readonly int $id;
}

class User extends Entity {
    public function __construct(
        public readonly int $id,
        public readonly string $name,
    ) {}
}

References

• ↗ https://www.php.net/manual/en/migration84.new-features.php

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