Furnace : undefined constant ?

Code:

[Server thread/CRITICAL]: ErrorException: "Use of undefined constant Furnace - assumed 'Furnace' (this will throw an Error in a future version of PHP)" (EXCEPTION)

I wrote this line in the PlayerInteractEvent.
When I tap on a chest(bc) or a trapped chest, it is working as normal.
However, the error comes up when I tap on a furnace. What is the error in a future version of PHP?

PHP:

use pocketmine\block\Chest as bc;
use pocketmine\block\TrappedChest;
use pocketmine\block\Furnace;


PHP:

if($event->getBlock() instanceof bc or Furnace or TrappedChest)


 

PHP:

if($event->getBlock() instanceof bc or $event->getBlock() instanceof Furnace or $event->getBlock() instanceof TrappedChest)


 

Or if you want make it if class exist use

PHP:

if($event->getBlock() instanceof bc or class_exists(Furnace::class) or class_exists(TrappedChest::class))