Have a look at the LEAP ORM for Kohana. It works with a bunch of databases, including DB2, Drizzle, Firebird, MariaDB, SQL Server, MySQL, Oracle, PostgreSQL, and SQLite. With a simple autoload function, it can work with almost any PHP framework. The source code is on GitHub at https://github.com/spadefoot/kohana-orm-leap. You can checkout LEAP's tutorials online.
The ORM library works with non-integer primary keys and composite keys. Connections are managed via a database connection pool and it works with raw SQL queries. The ORM even has a query builder that makes building SQL statements super simple.
