Tiki-generated pages can be compressed by Tiki's internal mechanism ($feature_obzip) or via PHP's zlib compression. "The zlib extension offers the option to transparently compress your pages on-the-fly, if the requesting browser supports this."
These two methods conflict, therefore Tiki compression will only be active if zlib is unavailable.
If you are getting blank pages or weird error messages, try disabling this feature. Please report any bugs.
On some servers, this feature conflicts with short URLS (via htaccess) (not sure how or why)
In TikiWiki, see "Use gzipped output:" on tiki-admin.php?page=general
As a note, this will compress output coming from php files, not css or js (which are actually static content), for these you could use deflate module (in Apache), and add mime types for .js and .css files, something like this:
In Debian, you find this file in /etc/apache2/mods-enabled/deflate.conf , and you just need to edit the file to suit your needs. This reduced the amount of data served for my tiki home page from ~800kB to ~200kB!