The Kohana PHP framework source (http://github.com/kohana/core) is a great example of well-written PHP code. You can definitely get a sense of the "good parts" of PHP from reading through the code.
Really? Perhaps I just looked at the wrong time, but I was looking at Kohana last year and ran far away. The issue wasn't the core, but everything else around it - the plugins, the dev attitudes picked up from mailing lists, the release process and more.
IIRC, I wanted to use Kohana because of some specific plugins recommended to me. However, they were marked as only working in 2.4. 2.3 was in beta, but the consensus I got from IRC people was 'just move to 3 - it'll be released soon and that's where attention is being focused'.
The core code might be well-written, but that doesn't mean it'd be a good project to build on. It might be, but code alone is not the only factor to consider.
