1. Related classes

A related class shares a logical connection or hierarchical connection with another class through inheritance or interface or composition

1. Inheritance Example

class Animal {
    public function makeSound() {
        echo "Some sound";
    }
}

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

2. Interface example

interface Logger {
    public function log($message);
}

class FileLogger implements Logger {
    public function log($message) {
        echo "Logging to file: $message";
    }
}

3. Composition exmple

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

class Car {
    private $engine;

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

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

2. Unrelated Classes

An unrelated class has no direct relationship — it does not extend from same class or implement from the same interface.

class Car {
    public function startEngine() {
        echo "Engine started";
    }
}

class Fruit {
    public function peel() {
        echo "Fruit peeled";
    }
}