Level Skip:

 Pause the game and go into the last  page of the passport. Once there,
 press Z, Y, Z, Y, X, X, X, Start. Then press C and you will skip ahead
 to the next level.

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All Weapons and Unlimited Ammo:

 Go to the inventory  screen and press X, Y, X, Y, Z, Z, Z, Z, Y, Z, Y,
 X, X, X, Start. You will hear a grunting noise. Now use the skip level
 code to beat the game.  Start a new game and punch in the gun code and
 then the skip. In the next level, you will have all the guns.

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Handstand:

 When Lara is hanging from a ledge, hold the  WALK  button and press UP
 to do a handstand.

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Unlimited Ammo:

 Enter the Level Skip code then press A + Up.