Infinite Ammo:
Start a new game, then hold L1 + L2 + R1 + R2 + Circle at the Weapons screen. You'll hear a sound to confirm.

Bonus Stuff:
Enter UTOPIA as a password. You now get all bonus missions, music and artwork in the Database screen.

Unlock All Missions:
Enter PLINTH as a password to unlock all 30 missions and credits

Thanks to Revolution reader Eria!