Thanks to Bill Abt and Rick McGuire of IBM, preliminary documentation for writing a PDO driver is now part of the PHP manual.

It was originally written against an earlier of incarnation of PDO, so some parts may not be 100% correct. If you're thinking of adding support for other databases to PHP, then you should read the howto and give it a whirl. If you find an inaccuracy, drop a line to pecl-dev@lists.php.net and we'll set you straight and fix the docs.