Readonly Classes in PHP 8.2

Since PHP 8.2, classes can be declared as readonly. This means that all class properties automatically becomes readonly. The readonly properties were introduced in PHP 8.1. These properties can be initialized once and cannot be modified later. Readonly classes are declared with readonly keyword.

Let's say we have User class, which has two readonly properties.

User.php

<?php

class User
{
    private readonly string $firstName;

    private readonly string $lastName;

    public function setName(string $firstName, string $lastName): void
    {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }
}

Since PHP 8.2, class can be rewritten by making it as readonly.

User.php

<?php

readonly class User
{
    private string $firstName;

    private string $lastName;

    public function setName(string $firstName, string $lastName): void
    {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }
}

For demonstration purpose, we created an instance of User class. A second call of setName method produces a fatal error because firstName property already set.

<?php

require_once 'User.php';

$user = new User();
$user->setName('John', 'Anderson'); // OK
$user->setName('Alex', 'Smith'); // Error

There are some notes for readonly classes.

Readonly abstract and final classes

Abstract classes and final classes can also be declared as readonly. The order of the keywords does not matter.

<?php

abstract readonly class Animal {} // OK

<?php

final readonly class SiberianHusky {} // OK

Readonly subclasses

If parent class is declared as readonly then child class must be declared as readonly as well.

<?php

readonly class Dog {}

<?php

readonly class Pomsky extends Dog {} // OK

<?php

class AlaskanMalamute extends Dog {} // Error