MySQL 5.1 has a plugin interface to easily add storage engines. PHP can easily embedded into other applications. So why not combine these two things? - Writing a MySQL Storage Engine which reads data by calling a PHP script.
He starts with a basic example with create_table and open_table functions that can pull the data from the table marked with the "Engine" setting of PHP. He expands on this to allow for more functionality - update, delete and write methods are added.