Unlimited Batarangs:
At the main options screen, press L1 R1 L2 R2, and you hear a sound confirming the code

All Weapons + Unlimited Ammo:
At the start Plant Electrocution 2 level, slide into the second hole and glide to the lit area and pick up the key. Use the grapnel to exit, continue the slide down and use the key on the door right next to the vines behind the glass. Inside you will find an envelope with a "C" on it. When you win the game, activate the cheat.