Unlock Cheat Menu:
Pause gameplay and press and hold R while you then press Down, Up, Square, Triangle, Triangle, Circle, Down, Up and Square and Triangle together. You'll know if you've entered the code correctly because when you now go to the 'Pause' menu the 'Cheats' menu will now be selectable.

Unlock ALL Artifacts and Keys:
Pause gameplay and hold L + R and press X, X, Circle, Square, Triangle, X.

Unlock Cheat Menu:
During game play 'Pause' the game and press and hold L and R, then press Down, Up, Square, Triangle, Triangle, Circle, Down, Up and then simultaneously press Square and Triangle. If you have entered the cheat correctly the cheats will be selectable at the 'Pause' menu screen.

Unlock Invincibility and ALL Weapons:
During game play 'Pause' the game, hold R and press Down, Up, Square, Triangle, Triangle, Circle, Down, Up, Square and Triangle. An extra category will unlock on the menu screen which if you select you will be able to turn 'Invulnerability' and 'Gain All Weapons' on and off.

Unlock Artifacts and Keys:
Press 'Start' and hold R and L and press X, X, Circle, Square, Triangle, X and a new option will appear at the 'Pause' menu.