If you are trying to connect to an SQL Server database with the dblib drivers for PDO, this is the correct connection string to use:
new PDO('dblib:host='.$host.';dbname='.$database, $this->user, $this->password);
You can also specify the encoding with
charset=ISO-8859-1 if necessary.
Be aware that dblib is incredibly buggy and that it does not support unicode or transactions. I have also encountered important other bugs, such as being unable to use
ERRMODE_EXCEPTION. However, there are not that many alternatives if you
want have to connect to SQL Server from PHP on Linux