If you get the error: TNS: could not resolve service name
Remember that the PDO wants to use the default client, which will only use the tnsnames.ora in %ORACLE_HOME%\network\admin. Check that file and make sure your service is defined in there.
BTW, there is a bug with pdo_oci8 with 9i - don't use it. Make sure you just use pdo_oci.dll.