Enter the following button combinations at the cheat screen:
Invincibility:
Press Left, Square, Up, Triangle, Right, Circle, Down, X
Unlimited ammunition:
Press Down, X, Left, Circle, Down, X, Left, Circle
Slow motion mode:
Press Right, Square, Up, Triangle, Right, Square, Up, Triangle
Invisibility:
Press Left, Square, Right, Circle, Left, Square, Right, Circle
Alien mode:
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle
Xen gravity:
Press Up, Triangle, Down, X, Up, Triangle, Down, X
Unlock Alien mode - Secret 10th mission:
Finish all 9 Decay levels with an "A" rank.
Unlock Bonus levels:
Go to the cheat screen.
Press Triangle, Square, Left, Right, Circle, X, Up, Down
Select the "Extras" menu in the options screen to load new levels found
from other discs, such as the Official US PlayStation Magazine Issue 57
(June 2002) demo disc.
Unlock Alien mode - Secret 10th mission:
Finish all 9 Decay levels with an "A" rank.
Save ammo:
Use guns to save ammo for other weapons. Kill the enemy with
the indicated numer of pistol shoots:
Crabs: 2 shots or 1 crowbar hit
Dogs: 3 shots
Electric Aliens: 3 shots
Machinegun on stand: 5 shots
Soldiers: 5 shots
Spitters: 4 shots
Trip Bomb: 1 shot
Zombies: 4 shots
