haphpiness

Interfaces, Traits, and Abstract Classes

PHP's OOP toolkit gives you three complementary tools for code organization: interfaces for contracts, abstract classes for shared structure, and traits for horizontal code reuse. Together, they solve the diamond problem without multiple inheritance.

// Interface: define the contract
interface Cacheable {
    public function getCacheKey(): string;
    public function getCacheTTL(): int;
}

// Trait: reuse implementation across unrelated classes
trait HasTimestamps {
    public DateTimeImmutable $createdAt;
    public ?DateTimeImmutable $updatedAt = null;

    public function touch(): void {
        $this->updatedAt = new DateTimeImmutable();
    }
}

// Abstract class: shared structure with extension points
abstract class Model implements Cacheable {
    use HasTimestamps;

    abstract protected function tableName(): string;

    public function getCacheKey(): string {
        return $this->tableName() . ':' . $this->id;
    }

    public function getCacheTTL(): int {
        return 3600;
    }
}

// Concrete: just fill in the blanks
class User extends Model {
    use SoftDeletes;  // Another trait — compose freely

    protected function tableName(): string {
        return 'users';
    }
}

The combination is powerful: interfaces ensure interoperability, abstract classes provide sensible defaults, and traits let you compose behavior horizontally without deep inheritance hierarchies. Modern PHP codebases are flat and composable, not deeply nested.