this is how I solved the issue with no firebird working in xampp,
it may be of interest since the xampp-tarball contains a interbase.so.

phpinfo() told the build env, --with-interbase=shared,/opt/interbase.
so...
ln -s /usr/lib/firebird/2.0 /opt/interbase (i run ubuntu)
ln -s /usr/lib/fbclient.so.2.0.3 /opt/interbase/lib/libgds.so
(and of course, enabling interbase in php.ini)
And everything was fine.

Maby already solved, still, quick and up.