All Weapons:

Enter the map mode by pressing Square+Select. Now press Left on the D-pad until the arrow is pointing at you. Press Start to return to standard view and, while the camera is rotating, press L1+Square+X+O simultaneously. Then, press Select to get out of Map mode. When you press Triangle you will have all of the weapons.

Invulnerability:

First, you need to have the drill. Next, press Triangle to bring up the weapons menu. Use L1 or R1 to scroll to and select the Frying Pan by pressing Triangle. Now, press Triangle again to bring up the weapons menu again. Press O+R1 until you get a flashing '999' in your health counter. Press O+R1 again to cancel invulnerability.

Full ammo:

Do a backflip by pressing Square + L2. While flipping, press Right + X + O.

No Collision Detection:

First, make sure you have your jetpack on. Next, find a dead body and stand on it. Press Triangle to bring up the weapons select screen and press L1+Right/Down. If the code works, you will immediately fall through the floor. To turn collision detection back on, press Triangle and press L1+Right/Down while in the weapons select screen again.

Level Select:

Go to the Main Menu screen and press L1+L2+R1+R2+Up simultaneously, then let go. Next, select a new game. Press L1+L2+R1+R2+Down at the same time, then let go. Select the difficulty level. A level select menu will pop up with all the levels available.

View ending:

Load a saved game. Hold Right, and then press Circle. Release Right and press Triangle to cancel. Next, press Left + Square, and then press Triangle to cancel again. You will now see the ending sequence.