"ghost" blocks

I am wanting to make a mario style block that allows the player to jump through the bottom but land on the top.

PPPPPPPPPPPPPLLLLLLLLLLLLLLLEEEEEEEEEEEEEEEAAAAAAAAAAAAAAASSSSSSSSSSSSEEEEEEEEEEEEEEEE HELP ME!!!!!!!!!

Please give more details, like what you have already tried, what engiene it is for, and what language.

umm forgot what ive done
engine : game maker 8
languige(can't spell) : GML or Game Maker Languige

Set it up so the player jumps at a speed, lets say 5px, then falls at a slower speed, like 3px, and set the platform to be 4px thick.