configure php :
#!/bin/bash
CC="gcc" OPTIM="-O2" \
./configure \
--includedir=/opt/informix/incl/esql \
--with-apache=/www/install/apache_1.3.11 \
--with-informix=/opt/informix \
--with-zlib \
--enable-sysvsem \
--enable-sysvshm \
--enable-track-vars \
--enable-magic-quotes

a piece of php3.ini :
[Informix]
ifx.default_host                =               ; default host for ifx_connect() (doesn't apply in safe mode)
ifx.default_user                =               ; default user for ifx_connect() (doesn't apply in safe mode)
ifx.default_password            =               ; default password for ifx_connect() (doesn't apply in safe mode)
ifx.allow_persistent            =       Off     ; allow or prevent persistent link
ifx.max_persistent              =       -1      ; maximum number of persistent links. -1 means no limit
ifx.max_links                   =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit
ifx.textasvarchar               =       0       ; if set on, select statements return the contents of a text blob instead of it's id
ifx.byteasvarchar               =       0       ; if set on, select statements return the contents of a byte blob instead of it's id
ifx.charasvarchar               =       0       ; trailing blanks are stripped from fixed-length char columns. May help the life
                                                ; of Informix SE users.
ifx.blobinfile                  =       0       ; if set on, the contents of text&byte blobs are dumped to a file instead of
                                                ; keeping them in memory
ifx.nullformat                  =       0       ; NULL's are returned as empty strings, unless this is set to 1. In that case,
                                                ; NULL's are returned as string 'NULL'.