Hold L1&R1 during gameplay then press the button combination. Do the same thing to turn it off.

Triangle, Triangle, Down, Down - Infinite Weapons

Up, Down, Left, Right, Right, Left, Down, Up - Invulnerability

X, X, Up, Up - Killer Weapons (Super Damage)

X, Triangle, X, - Triangle Mega Guns

Left, Left, Up, Down, Left, Right - Radial Blast

Triangle, X, Square, Circle - Trade Weapons for Health