I can answer for connecting to Informix on AIX from PHP loaded on Windows. I have no experience with PHP on Windows.
Regardless, you need the Informix SDK installed on your AIX system. For PHP and Apache on your AIX system, follow the Apache Module Quick Reference section (Chapter 2, page 53) in the PHP manual. When you configure PHP make sure you add "-with-informix=$INFORMIXDIR" (INFORMIXDIR is set to whatever directory Informix is in. Also, ensure you have $INFORMIXDIR/bin in your PATH).
Hank