Configure PHP with :
./configure  --prefix=/export/php3 --with-pgsql=/export/pgsql --enable-track-vars --with-apache=../apache_1.3.4 --with-informix --with-gd=../GD-1.18
make ; make install
cd ../apache_1.3.4
./configure --prefix=/export/apache --activate-module=src/modules/php3/libphp3.a
make then fails on the main httpd linking stage with :

 Undefined                       first referenced
 symbol                             in file
ifx_module_entry                    modules/php3/libphp3.a(internal_functions.o)
ld: fatal: Symbol referencing errors. No output written to httpd

[I'm using : 2.8.1 and Infomrix ClientSDK V:2.10.UC1 [eSQL 9.16.UC1]]
The httpd build line if :

cc  -DSOLARIS2=250 -DUSE_HSREGEX `./apaci`    \
      -o httpd buildmark.o modules.o modules/php3/libphp3.a modules/standard/libstandard.a main/libmain.a ./os/unix/libos.a ap/libap.a regex/libregex.a  -Lmodules/php3 -L../modules/php3 -L../../modules/php3 -lmodphp3 -lgdbm                -L/export/pgsql/lib -lpq         -L/export/informix/lib -L/export/informix/lib/esql -lifsql -lifasf -lifgen -lifos -lifgls -lnsl -lsocket -laio -ldl -lelf /export/informix/lib/esql/checkapi.o -lifglx       -lm -ldl -lsocket -lnsl  -lresolv  -lsocket -lnsl -ldl