Press Up, Down, Left, Right, Down, Up at the main menu.
At the mission briefing screen, press [Left, Right, Up, Down] five times.
At the mission selection screen, press Up, Left, Down, Right, Up, Right, Down,
Left, Up, L1 + R1, L2 + R2. If you entered the code correctly, the screen will
flash. All missions will be unlocked, and when the game is paused a "Cheat
Screen" option will appear on the bottom of the menu with such selections
as "Player Can't Die", "All Weapons", and "One Hit Kills".