PEAR::MDB2_Schema enables users to maintain RDBMS independant schema files in XML that can be used to create, alter and drop database entities and insert data into a database. Reverse engineering database schemas from existing databases is also supported. The format is compatible with both PEAR::MDB and Metabase.
MDB2_Schema is a well engineered, mostly stable package. It has multiple contributors all with a fairly consistent coding style.
The code is largely self documenting, and the only real problem is that this sometimes leads to it being hard to understand (due to its sheer verbosity).
The PHP documentation is reasonably complete, but unit test coverage is a little bit low.