Got it!
There was some creative linking going on in /usr/lib64 on the problematic system:
lrwxrwxrwx 1 root root 32 Mar 3 2018 libfbclient.so -> /opt/firebird/lib/libfbclient.so
lrwxrwxrwx 1 root root 20 Sep 14 2019 libfbclient.so.2 -> libfbclient.so.2.5.9
-rw-r--r-- 1 root root 898608 Aug 22 2019 libfbclient.so.2.5.9
lrwxrwxrwx 1 root root 38 Mar 3 2018 libfbclient.so.3.0.3 -> /opt/firebird/lib/libfbclient.so.3.0.3
...
lrwxrwxrwx 1 root root 32 Mar 3 2018 libgds.so -> /opt/firebird/lib/libfbclient.so
lrwxrwxrwx 1 root root 20 Sep 14 2019 libgds.so.0 -> libfbclient.so.2.5.9
Here, libfbclient.so.2 and libgds.so.0 both linked to libfbclient.so.2.5.9,
whereas libfbclient.so and libgds.so linked to whatever version is installed under /opt/firebird
On the 'good' system. it's:
lrwxrwxrwx. 1 root root 32 Nov 30 2019 libfbclient.so -> /opt/firebird/lib/libfbclient.so
lrwxrwxrwx. 1 root root 11 Nov 30 2019 libfbclient.so.2 -> libgds.so.0
-rw-r--r--. 1 root root 898608 Aug 22 2019 libfbclient.so.2.5.9
lrwxrwxrwx. 1 root root 38 Nov 30 2019 libfbclient.so.3.0.4 -> /opt/firebird/lib/libfbclient.so.3.0.4
...
lrwxrwxrwx. 1 root root 32 Nov 30 2019 libgds.so -> /opt/firebird/lib/libfbclient.so
lrwxrwxrwx. 1 root root 32 Nov 30 2019 libgds.so.0 -> /opt/firebird/lib/libfbclient.so
On this system, libfbclient.so.2 links via libgds.so.0 to /opt/firebird/lib/libfbclient.so
(i.e. current version), as do libfbclient.so and libgds.so. So here, we have nothing to
fear but libfbclient.so.2.5.9 itself.
After correcting the links on the 'bad' system and restarting httpd and firebird,
everything's fine there, php-boolean-wise.
BTW, this means that your driver loads libfbclient.so.2 or libgds.so.0 rather than
libfbclient.so or libgds.so. Is that intentional?
Cheers,
Paul