Enums Implementing Interfaces
debt(d3/e3/b3/t5)
Closest to 'default linter catches the common case' (d3). The detection_hints list phpstan and psalm as tools. Missing interface methods or incorrect return types are caught by these static analysers, which are near-default in modern PHP projects. The fatal error from trying to extend a class is caught at runtime instantly, but the more subtle mistakes (missing methods, wrong return types) need static analysis.
Closest to 'simple parameterised fix' (e3). The quick_fix describes adding `implements InterfaceName` to the enum declaration and implementing all required methods using `match($this)`. This is a small, localised refactor within one enum file — not a single-line patch but also not spanning multiple files.
Closest to 'localised tax' (b3). The choice applies to a specific enum declaration. While it affects all consumers of that enum through the interface contract, the structural commitment is bounded to the enum and its direct usages. It doesn't impose a cross-cutting burden on the broader codebase.
Closest to 'notable trap' (t5). The misconception field directly states the trap: developers expect enums to extend classes (as they might in other languages or with PHP classes), but enums can only implement interfaces because they already extend UnitEnum or BackedEnum internally. This is a documented gotcha that catches developers familiar with class-based OOP but new to PHP 8.1 enums.
TL;DR
Explanation
Pure and backed enums can both implement interfaces. This enables: type-hinted parameters that accept enums (function process(Colorable $c)), enum cases used in collections alongside other objects implementing the same interface, and enum methods satisfying interface contracts. Enums cannot extend classes (they already extend UnitEnum/BackedEnum internally), but implementing multiple interfaces is allowed. Interface constants can be declared in enums too (PHP 8.3+). Common pattern: Status enum implementing HasLabel interface with getLabel() method.
Common Misconception
Why It Matters
Common Mistakes
- Trying to extend a class with an enum — fatal error.
- Not declaring required interface methods in the enum body.
- Missing return types for interface methods in the enum implementation.
Code Examples
interface HasLabel {
public function getLabel(): string;
}
// Without interface — no type safety:
enum Status { case Active; case Inactive; }
function render($status) { /* no type hint */ }interface HasLabel {
public function getLabel(): string;
}
enum Status: string implements HasLabel {
case Active = 'active';
case Inactive = 'inactive';
public function getLabel(): string {
return match($this) {
Status::Active => 'Active',
Status::Inactive => 'Inactive',
};
}
}
function renderBadge(HasLabel $item): string {
return '<span>' . $item->getLabel() . '</span>';
}