I am currently trying to connect to my localdb on MSSQL 2012 Express.
I have downloaded and installed the official microsoft driver from http://www.microsoft.com/en-
Another possibility is to use the odbc drivers which are by default included in the php extensions, you still might have to uncomment them in your php.ini though.
extension=php_pdo_odbc.dll
Don't forget to restart your server afterwards ;-)
And then use it like this:
$db = new PDO('odbc:Driver={SQL Server};Server=192.168.x.x;Database=DatabaseName; Uid=User;Pwd=Password');
$stmt = $db->query("SELECT the_usual FROM aTable WHERE all='well'");