Ok, can't say I've actuallly tried this with jut php before but I could think of a few ways that should work, some will be more graceful than others.
One to try then, How about an array of files that you want to include and then use something like
<?php
$files = array("file1.html", "file2.txt", "file3.php", "file4.php");
$rand = array_rand($files);
include ($files[$rand]);
?>
You could include this in on each page or to make it more portable incase you change the array, include it in a class file or function and call it from there.
Forgive any syntax errors as I haven't tested it
Hope it helps
Regards
Sarah