I found the solution: while INFORMIXDIR was set in the registries, it also needed to be set in Apache's environment variables, along with a few others.

I added the followings to httpd.conf: 

#INFORMIX 
SetEnv INFORMIXDIR "C:/informix32"
SetEnv INFORMIXSERVER "server"
SetEnv DELIMIDENT n
SetEnv DBANSIWARN n
SetEnv CLIENT_LOCAL "en_US.CP1252"
SetEnv DB_LOCAL "en_US.CP1252"

[download]
and then restarted Apache.

In reply to Re^3: DBI connect('my_dsn','user',...) failed: [INTERSOLV][ODBC Informix driver][Informix]Unable to load locale categories. (SQL-HY000) by albi
in thread DBI connect('my_dsn','user',...) failed: [INTERSOLV][ODBC Informix driver][Informix]Unable to load locale categories. (SQL-HY000) by albi

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