My friend Kien and I have improved upon an earlier version of an ORM that he had written prior to PHP 5.3. We have essentially ported over Ruby on Rails' Active Record to PHP. It is still lacking some key features we want such as transactions, composite primary key support, a few more adapters (only MySQL and SQLite 3 work right now). But, we are very close to finishing this stuff up. You can take a look at PHP ActiveRecord with PHP 5.3.