I get the following error when trying to view some pages on linux server (SLES9.3, apache webserver and php configured with freetds).
Warning: mssql_query() [function.mssql-query]: WARNING! Some character(s) could not be converted into client's character set. Unconverted bytes were changed to question marks ('?'). (severity 16) in /www/htdocs/hrm/Lib/db_mssql.php on line 21
Line 21 in db_mssql.php says: $result = mssql_query($sql, $this->_connection) or die("query: ".$sql."
error: ".mssql_get_last_message());

Warning message doesn't always come up, and error raises only on several pages which all use the same db_mssql.php module to get resultset. Googling gave me the following results:

1. Check for variable multibyte non-UTF-8 input character set.
2. Use more robust error message generation.
3. For reads, cope with encodings that don't have the equivalent of an ASCII '?'.
4. Support alternative to '?' for the replacement character.

I'm not sure how to solve the problem, could you please give me some hints? Shall I examine MS SQL server collation, or make changes in freetds.conf? Note that the same web project running on apache under windows does not display this warning message, so I can say for sure that there is no problem with php code.

Thanks in advance for any comments you might have.