During gameplay pause the game and press Triangle and highlight the 'Help Text' selection and enter the following codes to unlock the corresponding effect. If the code has been entered correctly you will hear a sound as confirmation.
Unlock Full Health and Armor: Hold L1 + R2, and press X + Circle.
Unlock Full Ammunition: Hold L1 + R2, and press X + Triangle.
Unlock ALL Items: Hold L1 + R2 + R1 + Square and press X.