Some weeks ago I found a nice project, so just for fun I decided to make my version by using a S7 PLC. The lights are controlled by photocells (I used pcb buttons) at the ends of the stairs. Once someone braaks the beam the lights switch on from bottom to top or back.
A counting instruction based on double photocell prevents the lights from switching off when the stairs are still in use.
Surely a PLC is a bit expensive for a project like this. But you may change the syntax of the source code and make it available for
a PIC microcontroller by using this software. Please note that I don't know if and how it works, because I've never used it.
Here you can find a good tutorial on ladder logic, used on this project.