All Weapons:
Go to menu and hold:L1 and press X,square,X,circle,L2,R2,X a cheat option menu will appear.

Mission Select:
Press L3+R3 and press X to enter the "Mission Select" screen. This will not effect your memeroy card, unless you save it. With this code, you can get all the missions in the game. To exit the menu, press triangle.