Enums Implementing Interfaces

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>';
}

References

• ↗ https://www.php.net/manual/en/language.enumerations.interfaces.php

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