Why does PHP not save session variables for specific users with Internet Explorer?

Question

I have a problem with a website where PHP does not save session variables for specific users with Internet Explorer. But for some other users with Internet Explorer there is no

Answer 1

I figured out that the users that were having the problems all had Chrome Frame installed. I verified this by installing Chrome Frame on a local machine, and in this case I was able to replicate the problems.

The problems were caused by the fact that our server has Suhosin installed. The following Suhosin settings were enabled:

suhosin.session.cryptua
suhosin.cookie.cryptua

This means that the User Agent string is also a part of the identification of a user's session. Normally this is not a problem, but for users with the Chrome frame installed the User Agent string differs between the first request and the subsequent requests. After disabling these Suhosin settings there were no more problems.