Zeke is a skeleton
Hold X + Circle + A + Triangle at the opening sequence, before the
legal screen. Release the buttons at the title screen.

Zeke is invisible
Hold X + Square + L1 + R1 at the opening sequence, before the legal
screen. Release the buttons at the title screen.

Weapon upgrade
Hold Right, then press Square(2), Triangle(2), Circle(3), X during
game play.

Level select
Hold Right, then press Circle(2), Square, Triangle(2), Square,
Circle, X during game pla

Invincibility
Hold Right, then press Square, Circle(3), Square, Circle, Triangle,
X during game play.