Enable Cheat Mode:
The following location must be found to activate cheat mode. After the game starts, move to the right until reaching the location with a teddy bear. Face the back wall and press Circle to jump in that direction. A muted sound will confirm this portion of the code. Move forward and walk in the next room just to the right of this location, which leads down into a small room. Press Circle to access a platform. Use this platform to reach a playroom and enable cheat mode. The following codes may now be enabled.

All Weapons:
Enable cheat mode and during game play press Circle, Circle, Circle, Triangle, Triangle, X, X, Square on controller 2 .

Invincibility:
Enable cheat mode and during game play press X, X, Triangle, Circle, X, Square on controller 2.

Level Select:
Enable cheat mode and during game play press Triangle, Triangle, Square, Square, Square, X, X, Circle on controller 2.

Thanks to Revolution reader Jason Sommerfeld!