hehe.. thanks Ergin... I wish I'd known about that two months ago
I had tried unsuccessfully to get Interbase support in PHP working numerous times and utterly failed, the extensions said they were installed, phpinfo said they were installed, everything looked good except it just didn't work!
It seems that all the howto's out there assume you are connecting to a firebird server on another linux box, but I was actually trying to connect to an interbase server on a Wintel box, so I didn't have the required library files.
In the end I used unixODBC to do the job, and when the Database peeps decided to move to MSSQL, all I had to do was create the MSSQL driver, create a new DSN for it, and slightly alter my odbcConnect() function.
If anyone is still interested in a Howto: Connect to an MSSQL/Sybase server using unixODBC, please let me know