Properly using classes in other classes in php?

Question

Should have asked someone this a long time ago.

What is the best way to use other classes within another class?

For instance, lets say I have an application

Answer 1

$db could be a property of your Application class. Any reference to it from within an instance of Application would be done via $this - $this->db

class Application {

  private $db = null;

  public function setDB($name) {
    $this->db = new Database($name);
  }

}