Since many distributions do not compile InterBase support into php, it's a good idea
to check out how to achieve this. If you do not have the php sources, get them
here.
Untar them in your prefered sources location and configure them like this:
Make sure you give the right InterBase root directory as option. (/opt/interbase here)
Then issue:
When installing is finished, open the php.ini configuration file (located in /usr/local/lib/),
and check if these lines exist (uncomment/modify them if necessary):