Abstract Class

When to choose abstract class vs interface

• Abstract class — when subclasses share code (shared logic) and have an "is-a" relationship. Cannot implement multiple
• Interface — when a contract without implementation is needed; a class may implement multiple interfaces

Example

abstract class Shape {
  abstract public function area(): float;

  public function describe(): string {
    return "Area: " . $this->area(); // shared code
  }
}

class Circle extends Shape {
  public function area(): float { return M_PI * $this->r ** 2; }
}