On my Windows box I found a couple of things needed changing in my php.ini to get the PHP Data Objects talking to a MySQL database.

  1. enable extension_dir = "ext" in php.ini
  2. enable extension=php_pdo_mysql.dll in php.ini
In summary this tells PHP to look in the "ext" directory for "php_pdo_mysql.dll" where the "ext" directory is a subdirectory of your main PHP one.