composition.php

<?php

/**
 * This is our contract
 */
interface Service 
{
   public function doSomething();

   public function doSomethingElse();
}

/**
 * This is one of our SDK classes
 */
class ComplicatedService implements ServiceInterface
{
   public function doSomething()
   {
      // custom logic
   }
   
   public function doSomethingElse()
   {
      // more custom logic
   }
}

/**
 * If a consumer wants functionality from one of our classes,
 * extending it brings lots of inflexibility because it binds
 * you statically to the implementation of the parent. You get
 * everything in the class, which is often not what you want.
 */
class App extends ComplicatedService
{
   public function doSomething()
   {
      return 'Some custom message ' . parent::doSomething();
   }
   
   public function doSomethingElse()
   {
      // I didn't even want this parent method! Do I override or ignore? I now have it
      // and it's already added an unnecessary constriction to my current class :(
   }
}

$app = new App();
$app->doSomethingElse(); // The consumer didn't actually want this method

/**
 * Scenario 2 is composition. Instead of inheriting everything en masse, I am 
 * more selective, picking and choosing which behaviour I want.
 */
class App
{
   private $service;
   
   public function __construct(ServiceInterface $service)
   {
      $this->service = $service;
   }
   
   public function doSomething()
   {
      return 'Some custom message ' . $this->service->doSomething();
   }
}

$app = new App(new ComplicatedService());
$app->doSomethingElse(); // throws undefined method exception - no side effects :)