I finally solved my 'cannot connect via TCP/IP' problem
by uninstalling the debian firebird2 [classic] packages and installing firebird1.5
from source, and subsequently to install the php4-interbase debian package
via apt-get (which requires firebird2-lib package - which seems odd but doesn't
seem to hurt either!)

thanks for everyone who tried to help me get it running.

recommendation for those needing to run firebird with php4 on a debian
machine (at least if you have the misfortune that the machine in
question is a 'plesk' machine):

don't install the firebird2 package classic - install firebird1.5 from
source instead.

rgds,
jochem