On the ThisLab website, they've posted their comparison of two PHP frameworks, both based on some of the same code - CodeIgniter and Kohana.

I began hunting for PHP frameworks ever since Ruby on Rails hit the street. [...] I did a lot of searches, and researches, and finally I was happy to see CodeIgniter.

CodeIgniter has progressed well and gained many web developers' trust, despite a few glitches. One of which was the spawn of the fork: Kohana.

He points out the Wikipedia page showing some of the differences but the real interest in this post is the more than complete table directly comparing the two frameworks (specific versions of each). Items compared on include supported databases, community, conventions, caching, web service support and unit testing integration.