Invincibility:
At the main menu, press L2, L1, R2, Circle, Square, Circle, Circle, L1, L2, L1.

All tanks:
At the main menu, press L1, L2, L1, Circle, Square, Circle, Circle, L2, L1, R2. All the tanks, including a super tank with unlimited weapons, will be selectable.

Bonus level:
At the main menu, press L1, L2, L1, L2, R2, R1, Square, Circle, Square.