Level select:
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While playing the game, press Left(4), L1, Right(4), R1 to unlock all levels.

Restore health:
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While playing the game, press Up(2), Down(2), Left, Right, Left, Right, Circle, 
X to spawn a medpak that will restore 25 points of health.

Refill ammunition for current weapon:
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While playing the game, press Triangle, Circle, X, Square, R1, Triangle, Square, 
X, Circle, R1 to spawn an ammunition cartridge for your current weapon.

No clipping mode:
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While playing the game, press Up(3), L1, Up(3), R1, Up(2).

Enemies do not target you:
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While playing the game, press Down(3), L1, Down(3), R1, Down(2).