readonly Classes (PHP 8.2)

TL;DR

Explanation

readonly class Money { } makes all declared properties implicitly readonly — no need to mark each one. Rules: all properties must be typed. Cannot have untyped or static properties. Allows constructor promotion. Can extend readonly classes. Cannot extend non-readonly classes (or vice versa — asymmetric readonly inheritance breaks the contract). readonly classes implement all readonly property restrictions: write-once, no unset, requires type. PHP 8.3: readonly properties can be modified in __clone() for copy-with patterns. Ideal for: DTOs, value objects, request/response objects, command/query objects.

Common Misconception

Why It Matters

Common Mistakes

• Trying to extend a non-readonly class from readonly class — fatal error.
• Adding static properties to readonly class — not allowed.
• Assuming deep immutability — nested objects are still mutable.

Code Examples

class UserDTO {
    public function __construct(
        public readonly int $id,
        public readonly string $name,
        public readonly string $email,
    ) {}
}

// PHP 8.2 — readonly class:
readonly class UserDTO {
    public function __construct(
        public int $id,
        public string $name,
        public string $email,
    ) {}

    // PHP 8.3 copy-with:
    public function withName(string $name): static {
        $clone = clone $this;
        $clone->name = $name; // Only in __clone in 8.3
        return $clone;
    }
}

References

• ↗ https://www.php.net/manual/en/migration82.new-features.php

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