Easiest way would probably be to make an array of all of the filenames
$array[1] = 'filename.txt';
$array[2] = 'filename2.html';
Then do a count on the array
$num = count($array);
Then pick a random number between 0 and $num
$random = rand(0, $num);
Then include the filename listed at $array[$random]
include "$array[$random]";
I'm not great, but that should get you started (until infinity comes along with a better solution
Kevin