Good PHP ORM Library?

Is there a good object-relational-mapping library for PHP? I know of [PDO][1]/ADO, but they seem to only provide abstraction of differences between database vendors not an actual mapping between the domain model and the relational model. I'm looking for a PHP library that functions similarly to the way [Hibernate][2] does for Java and NHibernate does for .NET. [1]: http://www.php.net/manual/en/intro.pdo.php [2]: http://en.wikipedia.org/wiki/Hibernate_%28Java%29