This little trick will allow you to do all sorts of cool things in the game such as skipping a level, being invinicible, adding health or items, increasing your head size, and more. To do it, first pause the game. Then enter the following putton sequence:

Hold L2 and then press the following buttons in this order: Triangle, Circle, Triangle, Circle, Circle, Triangle, Left, Circle, Up, Down, Right, Circle, Left, Left, Triangle, Right, Circle, Left, Left, Triangle, Circle, Down, Circle, Circle, Right.

Once that is done, you will have access to the cheat menu, which is found in the pause screen.