I installed hollow core bifold doors on a closet, measured and drilled holes on front and back for screw that holds a pull knob, inserted screw in back hole, but can't get it to exit front hole. Any ideas?

Sounds like you need a longer screw. (strangely, that's the punch line for an off-color joke)

With a hollow core door, you'll also need a washer to keep the screw from pulling through the thin surface.

Yeah, the screw just clears the front layer and goes into the knob about 1/2". I had already decided to use washers, and may make the holes a bit larger, as they will be covered up anyway. Thanks!