Lesson 15 — Object-Oriented PHP

Inheritance and Interfaces

Inheritance lets a class extend another, reusing and specialising its behaviour, while interfaces define a contract of methods a class promises to implement.

Extending a Class

The extends keyword lets one class inherit another's public and protected properties and methods, and can override them with its own implementation.

PHP — Inheritance 
class Animal {
    public function __construct(protected string $name) {}

    public function eat(): string {
        return "{$this->name} is eating.";
    }
}

class Dog extends Animal {
    public function bark(): string {
        return "{$this->name} says Woof!";
    }
}

$d = new Dog("Rex");
echo $d->eat();  // inherited
echo $d->bark(); // defined in Dog

Interfaces

An interface declares method signatures without implementations. A class that implements an interface guarantees it provides those methods, letting different classes be used interchangeably wherever that interface is expected.

interface Payable { public function calculatePay(): float; } class Employee implements Payable { public function calculatePay(): float { return 5000.00; } }

Common Mistakes

  • Trying to instantiate an abstract class directly — abstract classes exist only to be extended.
  • Forgetting to implement every method an interface requires, which causes a fatal error.
  • Using inheritance to model a relationship that isn't truly "is-a", when composition would fit better.
  • Assuming PHP supports multiple class inheritance — a class can only extend one parent, though it can implement multiple interfaces.

Professional Tip

A class can implement multiple interfaces even though it can only extend a single parent class. This is PHP's way of supporting flexible, multiple "contracts" without the complexity of true multiple inheritance.

Your Turn

Create an interface called Shape with a getArea() method, then implement it in two classes, Circle and Rectangle, each with their own area calculation.

Mini Quiz

How many classes can a single PHP class directly extend?