During gameplay press 'Start' to pause the game. Then press the R1 button and enter the following codes to activate the effect you want in the game. When you've entered a code correctly a description of the code will appear at the bottom of the screen as confirmation. Cheats that don't affect a players stats are only enabled on the level they were entered.
Unlock Full Ammo: Press Up, Left, Triangle, Down.
Unlock Full Health: Press Down, Down, Up, Up.
Unlock Rapid Fire Mode Press Down, Down, Down, Down.
ALL Skills Max'ed: Press Left, Up, Right, Down.
Unlock Extra Skill Points: Press Up, Up, Down, Down.
Health Limit Increased: Press X, Circle, Left, Left, Right.
Invulnerability for the Player: Press Triangle, X, Square, Circle.
Invulnerability for your Allies: Press Triangle, Circle, X, Square.
Kill ALL the Enemies in the Level: Press Down, Up, Triangle, Right.
Player gets a Large Gun: Press Left, Right, Square, Circle.
Player gets ALL German Weapons: Press Up, Down, Left, Right, X.
Player gets ALL US Weapons: Press Up, Down, Left, Right, Square.
Player gets ALL UK Weapons: Press Up, Down, Left, Right, Circle.