To unlock four extra gameplay modes, you have to find all Enigma codes in the game. Then, in the main menu, select "Extras", then "Enigma codes".
To unlock the first game mode, set the lines this way:
The first mode will give you unlimited ammo, much more health, and you'll play on a new difficulty level.
To unlock the second new mode, set the lines this way:
This mode will delete health, ammo and armor levels, and you'll play on a new difficulty level.
To unlock the third new mode, set the lines this way:
This mode will delete all first aid kits and armor parts, and you'll fight on a new difficulty level.
To unlock the fourth new mode, set the lines this way:
This mode will give you only one life. Moreover, if you die, you won't be able to restart from a save point. You'll also play on a higher difficulty level.