Ok it dosent matter which level you start on all you have to do is go to your pda then hold the left trigger and hit x,y,b,a you might have to do it twice but after you do it your pda should go off and you will be invincible!!

On the last level just before you get to the boss turn left and somewhere you will see a brick saying id under a ledge [crouching] click on id and another room will appear with a PDA when you read it, it shows the game designers saying thanks to everyone!

To get invincibilty, you do not need to open the PDA, but you do use X,Y,A,B. IF you do it right and in a quiet area, you'll hear a confirmation sound.

To Be Invincible hold sprint(L)and press A,X,Y,B

To skip checkpoint hold down sprint(L) and press B,A,X,Y

On any level, hold down L, and press X,Y,B,A, and you'll have infinite health.

I would like to correct my earlier chet that is to skip level (SORRY!)
This is how to get it
Go to PDA,go to objective screen,hold L trigger and press X,Y,B,A

Go to your PDA,go to the objective screen, hold L trigger and press B,A,X,Y then test it out if it doesn't work do it again

I'm not sure if this one works but the cheat for invisability is=hold left trigger and press x y b a while playing the game it should work

Have fun cheating :)

enter your PDA hold down the left trigger and press X,Y,B,A you may need to do this once or twice and you will need to do it over again if you finish a level