From the looks of your configuration, you're trying to connect to a MySQL database while your GunzDB is MSSQL.
Code:
$bdd = new PDO('mysql:host=PC-NAME/SQLEXPRESS;dbname=GunzDB', 'sa', 'password');
I wouldn't try to use "mssql" as a replacement in that red text above. But someone said something about uncommenting something from "extension=php_pdo_mssql.dll" line from your php.ini. Have not tried this yet. Care to test it out if you wish. It seems to be a lot of work to get PDO to work with MSSQL instead of MySQL.