Invincibility:
Press L1 L2 R1 R2 Square and left on the D-pad and press start and you will see the words "god mode on."
No Gravity:
Press L1 + L2 + R2 + Left + Square all at the same time then hit start.
All Explosive:
Press L1 + R1 + Square + Left at the same time then start.
Heavy Guns:
Press L2, R2, Square, Left at the same time then start.
All Ammo:
Press select then hold R1 Square and Left together and press start. (WARNING!! this cheat gets rid of new weapons that you picked up or selected when being briefed on your next mission and leaves you with the weapons you have in the first level)