I have finally gotten the Interbase extension working with Firebird.
I used, on Linux:

php-4.3.3RC3
Firebird CS 1.5RC5.tar.gz

I install Firebird first, using the simple install script.
Then I built php using --with-interbase=/usr/local/firebird as an
argument for ./configure

Initially, I tried --with-interbase=shared,/usr/local/firebird
but the interbase extension never got enabled, as far as PHP was
concerned, even though I modified my php.ini to enable interbase.so.
Strange.

Nevertheless, it's working.