I installed PHP on windows IIS using Windows Platform Installer (WP?). WP? creates a "PHP Manager" tool in "Internet Information Services (IIS) Manager" console. I am configuring PHP using this tool.
in http://php.net/manual/en/pdo.installation.php says:
PDO and all the major drivers ship with PHP as shared extensions, and simply need to be activated by editing the php.ini file: extension=php_pdo.dll
so i activated the extension using PHP Manager and now PDO works fine
PHP manager simple added the following two lines in my php.ini, you can add the lines by hand. Of course you must restart the web server.
[PHP_PDO_PGSQL]
extension=php_pdo_pgsql.dll