Reasons why gzipped content might not be grokked by the browser

You could also simply use Django's GZip Middleware:

Either by enabling the middleware in settings.py by adding:

MIDDLEWARE_CLASSES = (
    django.middleware.gzip.GZipMiddleware,
    ...
)

Or do it before you return a particular response. In your views.py, dec would be the handler for a certain url

from django.middleware.gzip import GZipMiddleware

gzip_middleware = GZipMiddleware()

 def dec(request, *args, **kwargs):
        response = func(request, *args, **kwargs)
        return gzip_middleware.process_response(request, response)
        return dec

NOTE: You should be certain you are not subject to side-channel attacks before using GZip middleware.

Warning

Security researchers recently revealed that when compression techniques (including GZipMiddleware) are used on a website, the site may become exposed to a number of possible attacks. Before using GZipMiddleware on your site, you should consider very carefully whether you are subject to these attacks. If you’re in any doubt about whether you’re affected, you should avoid using GZipMiddleware. For more details, see the the BREACH paper (PDF) and breachattack.com.

Also:

Changed in Django 1.10: In older versions, Django’s CSRF protection mechanism was vulnerable to BREACH attacks when compression was used. This is no longer the case, but you should still take care not to compromise your own secrets this way.

That's not really the answer you might expect, but did you try not compressing out data with PHP, but with Apache ?

This can be done use mod_deflate.

Couple of things :

• it would mean less calculations for PHP
• it would also mean (and that's the nicest part ;-) ) that your CSS/JS files could be compresses to ; considering the size of those, nowadays, it might not be a bad thing to compress them...

As a sidenote : to help us figure out what's going wrong with the compression by PHP, could you give more informations ? Like the URL of your site (if it's public), and some code ?

EDIT now that we have the URL : http://ego.pestaa.hu/

When I go to that page, the response's headers are as follow :

HTTP/1.x 200 OK
Date: Sat, 01 Aug 2009 21:53:37 GMT
Server: Apache
X-Powered-By: PHP/5.2.6
**Content-Encoding: gzip**
Vary: Accept-Encoding
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html

(using firefox 3.5 with LiveHTTPHeaders extension)

The three testing sites you indicated also say that your page is gzipped.
Did you find what the problem was ?

Are you sure this is not something coming from your browser, that would not send the following header in the request :

Accept-Encoding: gzip,deflate

EDIT after other answer

Quote from the comments :

the source of problem probably isn't my website but my isp/browser/router. One of them may decompress every component before it reaches internal processes.

Oh, if the problem is not your website, I think I totally mis-understood the question -- sorry about that :-( I thought you didn't know how to server gzipped content from your website.
And I think I'm not the only one, btw

If the problem has nothing to do with your website's configuration, then, two possibilities :

• maybe a mis-configuration of firefox ? If you go to about:config, what does the network.http.accept-encoding say ? For me, it's gzip,deflate
• if the problem is coming from your ISP or something like that, there's nothing anyone can do...