PHP RFC: Static class

• Version: 1.3
• Date: 2024-06-23
• Author: Paul Morris bilge@scriptfusion.com
• Status: Under Discussion
• Target Version: PHP 8.4
• Implementation: https://github.com/php/php-src/pull/14861
• First Published at: https://wiki.php.net/rfc/static_class

A static class is a class that cannot be instantiated, and whose members (properties and methods) are all static. Implicit static classes can be created today, by simply only including static members. However, explicitly marking a class as static has several advantages:

• Clear expression of developer intent.
• Language-level compile-time checks to verify no instance members are included anywhere in the inheritance chain.
• Language-level runtime checks to verify the class is never instantiated by any mechanism.

Of these advantages, the first can be somewhat mitigated with a userland trait, but as such traits are non-standard, there is no consistent way to identify them between projects. Moreover, the language-level checks cannot be so easily satisfied, thus are better suited for inclusion in PHP.

A static class is so marked with the existing static keyword, adjacent to the class declaration.

static class Foo {}

We introduce the static keyword at the class level to preclude the need to create a private constructor. That is, function __construct() is a compile-time error in a static class. Furthermore, attempting to instantiate a static by any means, whether with the new keyword, ReflectionClass::newInstance* methods or unserialize() hacks, is strictly forbidden and will result in a runtime error.

Whilst the goals of this RFC so expressed are fairly straightforward, we acknowledge there are many finer points to consider. In particular:

1. Should a static class imply final (as in C#)?
2. Should a static class prohibit inheritance (as in C#)?
3. Should a static class permit state?
4. Should traits be permitted in a static class?
5. Which magic methods should be supported?

To answer these questions, we consider the principal reason to mark a class static is to move from an implicit declaration to the explicit. Ergo, any decision precluding the developer from doing so is invalid, since it coerces them to avoid explicit static, defeating its purpose. Thus we derive the following razor:

We cannot remove features from a static class that would otherwise be present in a standard class.

Following this razor we arrive at the following design.

1. A static class may be marked final, but in the absence of such a mark, may still be extended.
2. A static class may extend other static classes, provided the parents are also marked static.
3. A static class may have static properties.
4. A static class may use traits, provided they introduce no instance members.
5. All static magic methods must be supported, i.e. just __callStatic().

One question our razor cannot answer is whether static member declarations still require the static keyword in a static class.

Marking a class readonly makes marking properties with the same optional, thus one might argue static should follow suit. However, I opine it should not. Unlike readonly, which typically applies to properties declared near the head of the class (by convention) where the same modifier is also applied, static members appear throughout the full length of the class body. That is, it should be apparent when jumping anywhere into a static class that its members are static, without having to check the class declaration. Therefore this RFC still requires the static modifier for all members regardless of whether the class is also marked static, to prevent them appearing as instance members. Aside, this is commensurate with the C# approach.

When static is applied to a class, the following other class-level modifiers become invalid and will raise compile-time errors.

• abstract — This modifier exists to denote a class cannot be instantiated, but the static modifier already carries this meaning.
• readonly — Read-only properties are only supported for instance properties, but instance properties are forbidden by static.

A static class only permits extending a parent class that is also marked static. A non-static class cannot extend a static class.

Anonymous classes are always instantiated and thus at odds with the concept of a static class. That is, the notion of an anonymous static class is inherently invalid and not supported.

Undeclared object properties can be added dynamically to object instances, but since static classes cannot be instantiated, there is nothing to attach such properties to. Static properties must always be declared explicitly.

A new ReflectionClass::isStatic method should be added, returning true when the class is so marked, otherwise false.

None known.

1. As noted in Readonly properties 2.0, read-only static properties are not supported due to a technical limitation. If that limitation should ever be lifted, we could revisit lifting the restriction on mutual exclusivity with the readonly modifier.
2. Static interfaces may be introduced later if there is such a demand.
3. Static traits may be introduced later if there is such a demand.

• A simple yes/no vote to include this feature.

• Static Classes and Static Class Members (C# Programming Guide)

It should be regarded as an error to use a static class as a type declaration, since they cannot be instantiated and thus the requirement can never be fulfilled by a matching instance. However, it is not technically possible to forbid such declarations within PHP itself because type checking is done when an instance is passed at runtime (and we can never have such an instance). The engine does not support type checking at compile time for function/method signatures. Such a check would be a good candidate for third party static analysers instead.

I would feel remiss not to give thanks to the following list contributors whose high quality feedback made significant contributions to this RFC.

• Mike Schinkel
• Alexandru Pătrănescu
• Claude Pache
• Everyone else who engaged with this proposal.

Thank you!