All Weapons:
During gameplay hit pause and then hold L1 and enter:
square circle up down x square

Level Select:
During gameplay hit pause and then hold L1 and enter:
triangle up x down
There will then be the level select option at the main menu

Debug Mode:
During gameplay hitpause, hold L1 and press:
down, down, triangle

Invincibility:
During gamepaly hit pause, hold L1 and press:
down, up, left, left, triangle, up, right, down

Infinite Lives:
During gameplay hit pause, hold L1 and press:
triangle, circle, X, square

Regain Health:
During gameplay hit pause, hold L1 and press:
square, X, triangle, circle
Thanks to StingRay84

All Weapons & Infinite Ammo:
Pause the game, hold L1 and press X, Triangle, Circle. If you entered the code correctly, you will hear a gunshot.

Endless Fall:
Pause the game, hold L1 and press X(6). If you entered the code correctly, you will hear a gunshot. There will be no end to the fall. Enter the code again to disable it