You can add a bunch of parameters to shape.php and spirograph.php

For example: shape.php?sides=4 draws a square

Similarly, spirograph.php?outsideradius=81&insideradius=43&distance=54 draws a spirograph with those parameters. Feel free to play around with the numbers.

If you aren't sure what the numbers correspond to (And unless you've created your own spirograph drawing code before, you have no reason to) then visit the Wikipedia page on these types of spirograph.