i wouldn't use the odbc functions. I would use the mssql functions. If you are on a unix box you will need to recompile php (--with-sybase) with freetds supoort (www.freetds.org). The MsSQL functions are not as complete as the MySQL functions, but still better than odbc.
If it is the windows version, you will already have a php module for connecting. You may have to enable it in your php.ini though, but i think it is by default (Been a long time since I used windows as a webserver).
If you must use odbc and on windows, you need to go into the control panel and set up a odbc connection first.
If you are on unix you have a few choices for odbc, just do a google search.