Simulate the action of a lift.
Use sections of the keyboard as the call buttons in the lift and on each floor of a building.
A simple block graphic diagram on a VDU screen can be used to show the movement of the lift in response to the keyboard.
Note: This problem is only solvable on systems with an accessible clock interupt.
PHP wasn't designed for this type of problem and I don't know enough about C++ to create it in that language. So I've gone back to my first programming language, Visual Basic, to create the solution.
Here's a screenshot:
The user presses the keys 1 to 4 to make the lift go to that floor. If the lift is in motion, the call is ignored. That's about it ;)