While "Options" is highlighted at the Main Menu, hold L1 + L2 + R1 + R2 to display the "Security Clearance" screen, with a password entry prompt for the "Invulnerable", "Infinite Ammo", "All Missions", and "All Bonuses" options. Hold X and press the D-pad Left or Right to change one of the characters in the desired password. Release X to input that character. Press Right to highlight the next character and repeat until the password is filled. Press Square to accept the password. If you entered the code correctly, the text will turn orange. Press Triangle to exit the Password Entry screen.
To unlock Infinite Ammo, you must enter "62ALMEIDA" as a code in the North American version of the game. Enter "ALMEIDA062" as a code in the European version of the game. Then, select the "Load" option and choose a previously saved file to continue the game.