$cd /usr/ports/database/php5-interbase/work/php5/ext/interbase
$./configure
{...}
checking for InterBase support... yes, shared
checking for isc_detach_database in -lfbclient... no
checking for isc_detach_database in -lgds... no
checking for isc_detach_database in -lib_util... no
configure: error: libgds, libib_util or libfbclient not found! Check config.log for more information.

$find /usr/local/ | grep libib_util
/usr/local/lib/libib_util.so
$find /usr/local/ | grep fbclient
/usr/local/lib/libfbclient.so