Unidentified function errors usually mean that you haven't complied in support for those functions into your PHP. You need to have some things setup to make PHP compile in the Informix functions and you need to configure PHP witht the --with_informix=yes flag. See http://www.php.net/manual/en/ref.ifx.php for full details about compiling PHP with Informix support.
If you believe that you've done this check phpinfo() to see if it's part of the configuration string. You may be running on an old configuration.