Interfaces and Abstract Classes

Interfaces define public contracts and permit many implementations. Abstract classes combine shared state or behavior with required abstract methods.

Practical example

<?php
interface Clock { public function now(): DateTimeImmutable; }
final class SystemClock implements Clock {
  public function now(): DateTimeImmutable { return new DateTimeImmutable(); }
}
final class AuditService { public function __construct(private Clock $clock) {} }

Keep interfaces cohesive and consumer-driven. Implementations must preserve behavior, not merely signatures. Use abstract classes only when shared protected mechanics are essential; interfaces plus composition reduce coupling and improve replaceability.