Visibility, inheritance, and interfaces

Encapsulation uses public, protected, and private. Inheritance shares behavior; interfaces define contracts without implementation.

Visibility

• public — callable from anywhere
• protected — class and subclasses
• private — declaring class only

Inheritance

class Admin extends User {
    public function canPublish(): bool { return true; }
}

Single inheritance for classes; traits mix in horizontal reuse. Override methods with compatible signatures; use parent:: to call parent implementation.

Interfaces

interface Notifier {
    public function send(string $message): void;
}

Classes implement interfaces; type-hint interfaces in constructors for testability (dependency injection).

Important interview questions and answers

1. Q: Abstract class vs interface?
   A: Abstract classes can hold state and partial implementation; interfaces are pure contracts—classes can implement many interfaces but extend one class.
2. Q: When use final?
   A: Prevent overriding a method or extending a class when invariants must not change.

Self-check

1. Can a class implement two interfaces?
2. Which visibility should internal helpers use?