There's also a FreeBSD version of the client libraries that'll work with PHP. It's not available for download from their website, but you can request a copy (cd or electronic delivery) by calling Informix Customer Support at 1-800-274-8184 option 3.
Some customer service reps know more about alternative operating systems than others, so you may have to do a bit of educating before they can locate the product in their database, but it's definitely there.
My e-mail confirmation referred to it as "Orderable Part Number 100-15871-204057-1", although that number may be specific to the electronically-delivered edition.