PHP RFC: Readonly classes

• Date: 2021-08-04
• Author: Máté Kocsis kocsismate@php.net
• Status: Accepted
• Target Version: PHP 8.2
• Implementation: https://github.com/php/php-src/pull/7305

PHP 8.1 added support for readonly properties via PHP RFC: Readonly properties 2.0. However, it's still not easy to declare (quasi-)immutable classes, especially if they contain many properties. Therefore, this RFC proposes to add support for readonly classes.

The usage of the readonly modifier added by PHP RFC: Readonly properties 2.0 is extended to classes:

readonly class Test {
    public string $prop;
}

Doing so will implicitly mark all instance properties of a class as readonly. Furthermore, it will prevent the creation of dynamic properties.

readonly class Foo
{
    public int $bar;
 
    public function __construct() {
        $this->bar = 1;
    }
}
 
$foo = new Foo();
$foo->bar = 2;
// Fatal Error: Uncaught Error: Cannot modify readonly property Foo::$bar
 
$foo->baz = 1;
// Fatal Error: Uncaught Error: Cannot create dynamic property Foo::$baz

PHP RFC: Deprecate dynamic properties added support for the #[AllowDynamicProperties] attribute which makes it possible to create dynamic properties without triggering errors. In order not to violate the read-only constraint, marking readonly classes with the above attribute is a compile-time error:

#[AllowDynamicProperties]
readonly class Foo {
}
 
// Fatal error: Cannot apply #[AllowDynamicProperties] to readonly class Foo

As neither untyped, nor static properties are covered by the Readonly properties RFC, readonly classes cannot declare them either:

readonly class Foo
{
    public $bar;
}
 
// Fatal error: Readonly property Foo::$bar must have type

readonly class Foo
{
    public static int $bar;
}
 
// Fatal error: Readonly class Foo cannot declare static properties

Similarly how overriding of readonly properties works, a readonly class can only extend a readonly parent:

readonly class A {}
readonly class B extends A {} // valid

But both of the following are illegal:

readonly class A {}
class B extends A {}
// Fatal error: Non-readonly class B cannot extend readonly class A

class A {}
readonly class B extends A {}
// Fatal error: Readonly class B cannot extend non-readonly class A

A ReflectionClass::isReadOnly() method is added, which reports whether a class is declared as read-only. Additionally, ReflectionClass::getModifiers() will also include the ReflectionClass::IS_READONLY flag.

None.

• https://github.com/php/php-src/issues/9285: It used to be possible to add non-readonly properties to readonly classes via traits. As on PHP 8.2 RC 1, traits cannot be used by readonly classes if they define any non-readonly property, otherwise a compilation error is emitted.

Voted started on 2022-04-27, ending on 2022-05-11