I am running Apache with mod_gzip-18.104.22.168a on Windows XP, and I get problems, such as undefined variables when I run a SQL query.
A tip from Jose Fandos: put a comment on the following two lines in httpd.conf, like this:
# mod_gzip_item_include file \.php$
# mod_gzip_item_include mime "application/x-httpd-php.*"
as this version of mod_gzip on Apache HTTP server (Windows) has problems handling PHP scripts. Of course you have to restart Apache.