<"Class '$className' do not inherit from Singleton!");
        }
      }
      return self::$instanceMap[$className];
    }

    //protected constructor to prevent outside instantiation
    protected function __construct(){}

    //denie cloning of singleton objects
    public final function __clone(){
      trigger_error('It is impossible to clone singleton', E_USER_ERROR);
    }
  }

  class Apple extends Fruit{
    protected $rndId;

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

    public function whatAmI(){
      echo 'I am a Apple(' . $this->rndId . ')<br />';
    }

    public static function getInstance(){
      //echo get_class();
      return Fruit::getSingleton(get_class());
    }
  }

  class GreenApple extends Apple{
    public function whatAmI(){
      echo 'I am a GreenApple(' . $this->rndId . ')<br />';
    }

    public static function getInstance(){
      return Fruit::getSingleton(get_class());
    }
  }

  $apple1 = Apple::getInstance();
  //var_dump($apple1);
  $apple2 = GreenApple::getInstance();
  $apple1->whatAmI();// should echo 'I am a Apple(some number)
  $apple2->whatAmI();// should echo 'I am a GreenApple(some number)
  $apple1 = Apple::getInstance();
  $apple2 = GreenApple::getInstance();
  //保证单例模式
  $apple1->whatAmI();// should echo 'I am a Apple(same number as above)
  $apple2->whatAmI();// should echo 'I am a GreenApple(same number as above)
  // $a = clone $apple1;// this should fail
  // $b = clone $apple2;// this should fail