Pause the game, then hold L and press Up(2), Down(2), Left(2), Right(2), Square, Triangle. If you entered the code correctly, "Can't touch this" will appear when you exit the start menu.
All weapon upgrades
Pause the game, then hold L and press Up(2), Down(2), Left, Right, Left, Right, X, Circle.
Infinite ammunition
Pause the game, then hold L and press Triangle(2), X(2), Square, Circle, Square, Circle, Right, Down.
Refill ammunition
Pause the game, then hold L and press Triangle(2), X(2), Square, Circle, Square, Circle, Down, Right.