interface iPost {
  public function find($id);
}

class Post implements iPost {
  public function find($id){
    return $id;
  }
}

class PostController {
  private $post;

  public function __construct() {
   $this->post = new Post();
  }
}

class PostController {
  private $post;

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

class PostController {
  private $post;

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