I'm not sure if this project is still active but I have recently developed it further to include 'X-wing' solutions of order 2, 3 (swordfish) and 4 (jellyfish). I've also fixed a problem that occurrs in php 5.2.8 due to changes in the _to_string operation. I'm not sure how to post the code here but I can email it to the author if interested. It still does not solve the 'hard' example but it now solves everything in 'Extreme Sudoku' by Wayne Gould.
Ghica van Emde Boas - 2009-02-06 08:51:21 - In reply to message 1 from Mike Diplock
This is the true spirit of open source!
Yes, I would like to post your new version!
Please send it to me and I will do that.
I did not look at the package for quite a while, it is a hobby project after all. But I am still planning to do also the last resort: brute force tree walking, so that it can really solve every puzzle. But I still think that its solution strategy should be humanly understandable, therefore brute force is not really interesting.