What OS are you running, what web server, etc... It's far easier to help you out knowing these things.
I'll assume Unix/Apache...
You probably need to recompile php using the --with-interbase switch. For a list of all the switches for the configure script, type ./configure --help To look for a particular switch, try ./configure --help|grep string to search for a string. So, ./configure --help|grep interbase should yield the configure switch needed for interbase. ./configure --help|grep mysql for mysql etc...