dependency injection primer

bad.php

<?php

class PetrolEngine {
  public function vroom() {}
}

class Car {
  private $engine = new PetrolEngine();

  public function goFaster() {
    $engine->vroom();
  }
}

$myCar = new Car();
$myCar->goFaster();

best.php

<?php

interface Engine {
  public function vroom();
}

class PetrolEngine implements Engine{
  public function vroom() {}
  public function addPetrol() {}
}

class DieselEngine implements Engine{
  public function vroom() {}
  public function addDiesel() {}
}

class Car {
  private $engine;

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

  public function goFaster() {
    $engine->vroom();
  }
}

$myEngine = new PetrolEngine();
$myCar = new Car($myEngine);
$myCar->goFaster();

$myOtherEngine = new DieselEngine();
$myOtherCar = new Car($myOtherEngine);
$myCar->goFaster();

better.php

<?php

class PetrolEngine {
  public function vroom() {}
}

class Car {
  private $engine;

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

  public function goFaster() {
    $engine->vroom();
  }
}

$myEngine = new PetrolEngine();
$myCar = new Car($myEngine);
$myCar->goFaster();