All codes start with r2,l2,up,down. Just add the following:
Level select:
square,triangle,circle(main menu)
God mode:
x,r2,l2,r1,l1
All weapons:
x,up,square,x
Full heal:
x,up,down
Slow mo:
x,up,l2
Nailgun mode:
x,l1,l1
Ali mode:
x,up,up
Low gravity:
x,l2,l2
Lethal charge:
x,r1,r1
Bomb mode:
x,up,l1
Megaforce:
x,r2,r2