To do this you must first play through the game normally until you have completed Subway A1 and reached Chapter 2: One Way Or The Other. At this point pause the game and press 'Select' to return to the 'Main' menu where you then press Up, Down, Left, Right, Up, Left, Down, Circle and choose the 'Load level' option. If you're playing without a Memory Card select 'Continue' and ALL the levels will become unlocked.
Pause the game and press L1(2), L2(2), R1(2), R2(2), Triangle, Circle, X, Square. After an auto-save this code will need to be re-entered to remain invincible.
Unlock ALL Weapons and Full Ammunition:
Pause the game and press L1, L2, R1, R2, Triangle, Circle, X, Square. You will also receive 8 pain killer pills.
Infinite Bullet Time:
Pause the game and press L1, L2, R1, R2, Triangle, X(2), Triangle. To disable this effect enter the code again.
Slow Motion Sounds:
Pause the game and press L1, L2, R1, R2, Triangle, Square, X, Circle.