Full ammunition:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Left, Triangle.

Extra ammunition:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Left(2), L1, L2.

One hit kill with firearms:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Down, R1.

Expert mode:
Select "Credits" at the options screen then press Down(3), Triangle, Down(3), Square, Left, Right for more challenging opponents.

Rapid fire:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Up(3), Down.

Instant puzzle solution:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Down(3), Up.

One hit kill with smack-jack, knife, or brass knuckle:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Down, L1.

Invincibility:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Right, Square.

All weapons:
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Up, Circle.