php session not working (as with cookies disabled), but cookies are enabled for the domain. What other possibilities?

I have a problem with a client, who cannot log in to our system. We have PHP-based B2B system, which uses cookies to store session-id.

The client cannot log in and is redirected to the login page, without any error message. He claims he have cookies enabled in his Firefox. Also, if he had cookies disabled, my javascript would detect this and would show him an red, very descriptive error info. He does not seen anything like this.

What else could have same effect on sessions like disabled cookies in browser? Are there any proxies which filters cookies? Any AV software, etc? What to look for?

Our login form works for any other user without problems.

Update: I can see in logs, that the guy have logged in successfully,开发者_如何学Go but I don't know yet what was wrong. I will update when I know more.

Update #2:

I called the man. It seems that it was some NOD32 antivirus settings, that caused problems, but the man could not tell me details. He called a guy from tech department and the guy soleved the problem.

Anyway, thank you all for responses.

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Make him install temporarily another browser (Opera, Chromium, etc) and make him try to log in on the new browser.

If it still doesn't work, you know the problem doesn't come from his firefox config or some funny extension.

Ask him to log in from another location; for example if it doesn't work at the office, make him try a connection from home, if possible.

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Sometimes, due to P3P (Platform for Privacy Preferences) settings the cookie gets reset and PHP session is lost. I have seen it happening in IE6. You can check on this http://www.mail-archive.com/php-general@lists.php.net/msg41743.html

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You may want to ask him to delete any cookies he has for your site, or empty his cookies completely.

I would never have suggested this if I hadn't run into this axact problem and fix on a popular PHP-based forum system (phpBB).

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The problem was probably in the settings of NOD32 antivirus, but I do not know the details.