At the "Press Start" screen, press Up(2), Down(2), Left, Right, Left, Right, Circle to unlock the Young Alex costume. If you entered the code correctly, you will hear the sound of a page turning.
Successfully complete the indicated task to view the corresponding ending:
Collect all photographs to view an extra scene after the credits.
Successfully complete the indicated task to unlock the corresponding costume:
Successfully complete the game. Then, start a new game session with the cleared saved game file, and search the Shepherd family's garage to find the Circular Saw.
Successfully complete the game with the UFO ending. Then, start a new game session with the cleared saved game file, and look on top of the footlocker in Joshua's room to find the Laser Pistol.