Informix makes several specialized relational database servers for Windows NT and UNIX. Like Oracle and Sybase, Informix products are intended for demanding situations. I can't begin to discuss the unique features of Informix database servers, but you can learn more about them at their home site <http://www.informix.com/>.
PHP includes support for two parts of the Informix API, ODS and IUS. The functions that begin with ifx_, such as ifx_pconnect, are part of ODS and should be available to you. The IUS functions begin with ifxus_, such as ifxus_create_slob. These will be available only if you have IUS libraries. Keep in mind that ODBC drivers for Informix are available, so you could pursue using PHP's ODBC functions instead.
Jouni Ahto, ...