I did a quick search for the error code. One guy says:
"That looks suspiciously like the error code I get when I haven't properly set my INFORMIXDIR and INFORMIXSERVER environment variables."
Two other people said to make sure that the informix server is running properly (have you tried to connect to it some other way?).
And the final one I found was from a guy who had the error and fixed it. His first post said it was working for a few days, then they rebooted the server and got that error, then a little later he wrote:
"Thanks
We found the bug, we need re-make the php and apache ( because we reinstall
Informix IDS) and remove all httpd in the PATH.
we have problems with PHPLIB`s db_informix.inc because we need do some
changes from the original version to get ready.
someone have a full operational version of db_informix.inc or some clue
about it."
I hope something in there helps =)