Go to the Press Start screen -> New Game and hold L1 + L2 + R1 + R2. Now you can use one of the followng cheat codes:
All Chapters: Down, Left, Down, Triangle, Down
Unlimited Ammo: Up, Left, Down, Right, Circle
Unlimited Shotgun: Right, Circle, Circle, Circle, Square
Make Enemies Weaker: Square, Left, Triangle, Up, Down
Unlimited Health: Square, Triangle, Left, Up, Right
Unlimited Armor: Up, Up, Up, Square, Down
Unlimited Adrenaline: Left, Right, Left, Circle, Square
Easy Shot Mode: Square, Square, Square, Down, Right