I have started a Blog about using SQL in PHP scripts on Google App Engine.

<a href="http://blog.herbert.groot.jebbink.nl">http://blog.herbert.groot.jebbink.nl</a>

Google App Engine uses BigTable as database, this is a key-value database. Compared with a relational database like MySQL it has limited functionality. pQp tries to solve this by translating SQL statments to JDO api calls.