Verifique, no php.ini
extension=php_pdo_mssql.dll
e
extension=php_pdo_odbc.dll
e tente com o PDO
$server = 'host';
$database = 'database';
$username = 'username';
$password = 'password';
$db = new PDO('odbc:Driver={SQL Server}; Server=' . $server . '; Database=' . $database . '; Uid=' . $username . '; Pwd=' . $password . ';');
$query = $db->prepare('...');
$query->execute();