PHP RFC: Final class constants

• Date: 2021-04-23
• Author: Máté Kocsis kocsismate@php.net
• Status: Implemented
• Implementation: https://github.com/php/php-src/pull/6878
• Target Version: PHP 8.1

Currently, class constants are always overridable by child classes. This causes a few smaller caveats:

First of all, the engine can't optimize class constant references when late static binding is involved, so it has to pessimistically assume that FOO is overridden in case of static::FOO or $this::FOO invocations.

What's more important is that class constants are not guaranteed to stay constant. Even if the declaring class always references them by using self::, doing so won't prevent a child class from changing their value or even their type, unless the parent is a final class. Although constants not being constants is usually considered only as a theoretical problem, being able to add a final modifier would make the intention explicit that child classes shouldn't try to override them (e.g. because the parent doesn't use late static binding).

A related interesting fact is that interface constants are already final:

interface I
{
    public const X = "i";
}
 
class C implements I
{
    public const X = "bar";
}
 
// Fatal error: Cannot inherit previously-inherited or override constant X from interface I

This leads to a weird inconsistency. By introducing an intermediate class, overriding still becomes possible:

interface I
{
    public const X = "i";
}
 
class C implements I
{
}
 
class D extends C 
{
    public const X = "d";
}
 
// No error

The final modifier can be added to class constants. Doing so prevents overriding of a constant:

class Foo
{
    final public const X = "foo";
}
 
class Bar extends Foo
{
    public const X = "bar";
}
 
// Fatal error: Bar::X cannot override final constant Foo::X

Besides, interface constants would become overridable by default, and the final modifier could be used to retain the original behavior.

interface I
{
    public const X = "i";
    final public const Y = "i";
}
 
class C implements I
{
    public const X = "c"; // Overriding I::X is possible
    public const Y = "c"; // Overriding I::Y is not possible
}
 
// Fatal error: C::Y cannot override final constant I::Y

A ReflectionClassConstant::isFinal() method is added in order to be able to retrieve if a constant is final.

None.

Voting started on 2021-05-19 08:00 UTC and ends 2021-06-02 08:00 UTC. The vote requires 2/3 majority to be accepted.

Add support for final class constants?
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Final result:	29	4
This poll has been closed.