Pause the gameplay and hold L1. Then you can enter one of the following codes to eneble the corresponding effect:
Unlimited health: Triangle, Square, Triangle, Square, Triangle, Circle, Triangle, Circle
Get All Weapons: Square, Circle, Down, Left, Circle, Circle
Mission unlock: Down, up, Left, Left, Circle, Right, Down, Square
Get Daywalker Difficulty Setting: Left, Circle, Up, Down, Square, Circle, X
Unlimited Rage: Left, Down, Left, Down, Right, Up, Right, Up
Unlimited Ammo: Left, Circle, Right, Square, Up, Triangle, Down, X