Hold L1 + R1 and press Square, Circle, Triangle, X(2) during game play to display the code entry screen. You can then enable one of the following codes to activate the corresponding cheat function. Enabling a code will disable game saves.
Der Flakturm: Sewer escape
In the final part of the Der Flakturm mission, when you have to escape through the sewer, you will meet about six of the hard to kill elite machine gun soldiers. It is almost impossible to defeat them all. It is much easier to run past them all. Make sure you have at least three health bars. Once you climb the ladder the game is all but over.
Full ammunition
Hold L1 + R1 and press Circle(2), Triangle, Square, X, Triangle during game play.
Full health
Hold L1 + R1 and press Triangle, Square(2), Triangle, X, Circle during game play.