PHP, PDO dsn-less connection to MSSQL 2k8 in CentOS

Hi everyone !
Can someone help me with PHP, PDO dsn-less connection to MS-SQL 2008
in Cent OS, please?

code:

      $dsn = "FreeTDS;Server=10.x.x.x;Database=mydb;Uid=user;Pwd=password";

      $pdo = new PDO("odbc:Driver=$dsn",
                    "dbuser",
                    "password"
                   );


error:

SQLSTATE[IM002] SQLDriverConnect: 0 [unixODBC][Driver Manager]Data
source name not found, and no default driver specified


thanks,
celso