The following is list of prerequisites and assumptions needed for writing a PDO database driver:
A working target database, examples, demos, etc. working as per vendor specifications;
A working development environment:
Linux: standard development tools, gcc, ld, make, autoconf, automake, etc., versions dependent on distribution;
Other Unix: standard development tools supplied by vendor plus the GNU development tool set;
Win32: Visual Studio compiler suite;
A working PHP environment version 5.0.3 or higher with a working PEAR extension version 1.3.5 or higher;
A working PDO environment (can be installed using 'sudo pecl install PDO'), including the headers which will be needed to access the PDO type definitions and function declarations;
A good working knowledge of the C programming language;
A good working knowledge of the way to write a PHP extension; George Schlossnagle's Advanced PHP Programming (published by Developer's Library, chapters 21 and 22) is recommended;
Finally, a familiarity with the Zend API that forms the heart of PHP, in particular paying attention to the memory management aspects.