Full ammunition
Pause the game and press Triangle. Then highlight the "Help Text" selection, hold L1 + R2, and press X + Triangle. If you entered the code correctly, you will hear a sound. Now select the "Return To Game" option to resume with full ammunition.

Full health and armor
Pause the game and press Triangle. Then highlight the "Help Text" selection, hold L1 + R2, and press X + Circle. If you entered the code correctly, you will hear a sound. Now select the "Return To Game" option to resume with full health and armor.