I have PHP accessing Informix 7.31 IDS databases on Redhat 6.0 (Yeah, I know, but Informix is touchy about glibc versions, etc. If it ain't broke, don't fix it.) and Solaris 7 (Intel) systems, but they only access the database on the local system. I've "played" with accessing Informix databases across the network but never had much luck with it. Since it was essential I let it go.
Why don't you just install Apache and PHP on the Solaris box? If you do I highly recommend using the GCC compiler to do your installations.
Hank