While the gameplay, hold L2 + R2 then press Right, Right, Down, Up, Left, Left to unlock all weapons.
Go to the setup menu and at the cheat codes option enter one of the following codes:
All Boxers: Start, R1, X, O, Square, R1, R1, R1, Start
Conversation Mode: Start, X, O, Square, Square, R1, R1, Square, O, X, Start
Double Speed: Start, R1 x10, Start
Intercontinental Mode: Start, X, X, X, O, O, O, Square, Square, Square, Start
Scrap-Yard Scene: Start, O, R1, O, R1, X, X, Start
Second Skin: Start, X, R1, O, Square, Start