How can I use PHP to include the most recent file in a directory?

Question

I have a directory which will look something like this:

/test/
  |
  |- index.php
  |- test-20110324
  |- test-20090901
  |- test-20070901

What I want to do is use "index.php" to find the most up-to-date file, and require it into the "index.php" file.

Please note: The included file needs to be selected by the date-stamp suffix, as opposed to the file modification time, as these older files my need to be edited-in-place, without taking precedence over the most up-to-date file.

This concept will be applied over several directories, but the HTML files will all following the pattern above, i.e. WORD-YYYYMMDD.

Any ideas on how to do this? I will be very grateful if anybody could spare a moment to help me out here!!!

Thanks in advance!