Use this tool to generate a php random html page script.
On the next page enter a base url and the relative urls for your html pages.
Then take the generated script and save it as a php document.

Then add the url for the file to a embed <embed src="" /> tag and add it to your sig box like any other embed.

Your base url should look something like this.
http://your_domain/sigs/

The relative urls for your sig pages would look something like this
sig1.html
Or if they are down another directory it would look something like this
holiday/sig1.html