Composition vs. Inheritance in PHP

When designing software applications in PHP, one of the most critical decisions developers face is how to structure their classes and relationships. Two fundamental principles of object-oriented programming (OOP) come into play here: inheritance and composition. Understanding the differences, strengths, and weaknesses of each can help you design more maintainable and scalable applications.

Understanding Inheritance

Inheritance allows one class (a child or subclass) to inherit properties and methods from another class (a parent or superclass). It’s a core OOP principle that promotes reuse and logical relationships.

Example of Inheritance

class Animal {
    public function makeSound() {
        return "Some generic animal sound";
    }
}

class Dog extends Animal {
    public function makeSound() {
        return "Bark";
    }
}

$dog = new Dog();
echo $dog->makeSound(); // Outputs: Bark

In this example, the Dog class inherits from Animal but overrides the makeSound method. This is a classic use of inheritance to model an “is-a” relationship (a dog is an animal).

Pros of Inheritance

• Code Reuse: Shared functionality in the parent class reduces duplication.
• Natural Relationships: When an “is-a” relationship exists, inheritance models it cleanly.
• Polymorphism: Parent classes can define interfaces that child classes implement differently.

Cons of Inheritance

• Tight Coupling: Changes in the parent class can inadvertently affect child classes.
• Single Inheritance Limitation: PHP only supports single inheritance, which can lead to rigid designs.
• Overuse: Over-reliance on inheritance can lead to deep class hierarchies, making the system harder to maintain and test.

Understanding Composition

Composition involves building classes by combining smaller, reusable components rather than extending existing classes. Instead of “is-a,” composition models a “has-a” relationship.

Example of Composition

class Engine {
    public function start() {
        return "Engine started";
    }
}

class Car {
    private $engine;

    public function __construct(Engine $engine) {
        $this->engine = $engine;
    }

    public function startCar() {
        return $this->engine->start();
    }
}

$engine = new Engine();
$car = new Car($engine);
echo $car->startCar(); // Outputs: Engine started

Here, the Car class uses an instance of the Engine class. The car “has-a” engine, and this relationship is modeled through composition.

Pros of Composition

• Flexibility: Components can be swapped or reused across multiple classes.
• Loose Coupling: Changes to one component don’t necessarily affect the rest of the system.
• Better Scalability: Encourages the design of smaller, modular classes.

Cons of Composition

• More Boilerplate: Requires explicit instantiation and wiring of components.
• Complexity: Can lead to more moving parts, which might be harder to understand for simpler use cases.

When to Use Inheritance vs. Composition

Deciding between inheritance and composition depends on the problem you’re solving:

• Use inheritance when:

  • The relationship is genuinely an “is-a” relationship.
  • There’s significant shared behavior that won’t change often.
  • You need polymorphism for substitutable behavior.

• Use composition when:

  • The relationship is a “has-a” or “uses-a” relationship.
  • Flexibility and loose coupling are priorities.
  • You want to favor smaller, reusable components.

General Best Practices

• Favor Composition Over Inheritance: This is a core principle of modern OOP design. Composition tends to be more flexible and easier to extend or refactor.
• Keep It Simple: Avoid deep inheritance hierarchies or overly complex compositions.
• Consider SOLID Principles: Especially the Dependency Inversion and Open/Closed principles, which align well with composition.