Unscrew the loose knob from the inside of the cabinet. Turn the Phillips head screwdriver counterclockwise. Look to see if the screw is too short; you should be able to see at least 1/2 an inch of the screw sticking out from the other side of the cabinet door. This is the reason why the knob keeps coming loose.

Place the Phillips Head screwdriver onto the head of the screw and turn it counterclockwise.