Start game as usual, in game - bring up the console with ~
Now input the following codes:
PKGOD - toggle 'godmode' on/off
PKDEMON - toggle 'demon morph' on/off
PKWEAKENEMIES - toggle 'enemies get 1 hp' on/off
PKALWAYSGIB - toggle 'excessive gibbage' on/off
WEAPONSPECULAR 1/0 - toggles 'weapon specular' on/off
PKWEAPONS - gives all weapons
PKAMMO - full ammo
PKHEALTH - full health
PKPOWER - full ammo/health
PKGOLD - lots of gold
PKHASTE - toggle haste on/off
PKWEAPONMODIFIER - toggle 'weapon modifier mode' on/off
PKKEEPDECALS - toggle 'decals will never disappear' on/off
PKKEEPBODIES - toggle 'bodies will never disappear' on/off
Note:
Cheats are active only in Daydream and Insomnia difficulty modes!