Today while trying to create the interbase.so object for php under a Red Hat server I found myself with the error _configure: error: libgds, libib_util or libfbclient not found_. After 2 hours of trying to configure PHP with different versions of Firebird I found the one that worked was version 2.0.4 with ntpl threading support. I don’t know how did I missed that!.

From there upgrading to Firebird 2.1 was painless and everything is working as expected now. Hope is helpful for you guys.