I had the same problem.
Find and install the Install the "flex" RPM. You may hit
some dependency problems as you do it, theyy need to be
solved too.

Alternatively I can make available to you my interbase.so,
or I can post it in some public area indicated by you.

I compiled my interbase.so under Fedora C2 and PHP 4.3.5. I
tried 4.3.6 but it failed so I decided to be happy with
what worked. I believe it should work under RH 9. The size
is 128KB uncompressed.



I can now ./configure OK but hen I try to make I get a string errors about
undelcared identifiers and no such file or directory.. I've deleted the
configure cache file and I get a clean configure. I've tried to find if it's
a missing apxs tool but I can't find if there is a separate rpm for this. I
do have mod_so.c listed and I've installed bison.
I don't suppose you have any other clues about what I need?
I've had to install firebird via the tar ball - the rpm won't work at all -
it's up and running OK.
I've got apache 2.0 - not sure if that has anything to do with it - apache 2
comes with RH9.

Alan