Invincibility
     At  the  Cheat  Code  screen,  press  Left, Square, Up, Triangle,
Right, Circle, Down, X, Start.

Unlimited Ammo
     At  the  Cheat Code screen, press Down, X, Left, Circle, Down, X,
Left, Circle, Start.

Unlock Alien Mode
     At  the  Cheat Code screen, press Up, Triangle, Up, Triangle, Up,
Triangle, Up, Triangle, Start.

Unlock Invisibility
     At  the  Cheat  Code  screen,  press Left, Square, Right, Circle,
Left, Square, Right, Circle, Start.

Unlock Slow Motion Mode
     At  the  Cheat  Code  screen,  press Right, Square, Up, Triangle,
Right, Square, Up, Triangle, Start.

Unlock Xen Gravity
     At  the  Cheat  Code  screen,  press  Up,  Triangle, Down, X, Up,
Triangle, Down, X, Start.