As mentioned above, PHP on GAE just went to general availablility, and there are definetly no plans to turn down this service.
Currently, we do all our development inside of Google and not in github. The two major peices we work on are
b) The GAE PHP PECL extension. This code is currently not open sourced although we are looking at putting it on github.
https://github.com/GoogleCloudPlatform/appengine-php contains very few GAE specific changes from stock PHP, and therefor we infrequently update the github repository. Ultimately this code is not that interesting for GAE developers when compared to (a) and (b) above.
As for the blog - I just haven't had time to write any interesting articles. If you can think of anything worth adding to the blog, please let me know.
Regards,
Stuart