We were just struggling here at work with an insane problem where IE 7 (and ONLY IE 7) was dropping sessions for PHP. Literally, we would try a trivial test case of creating a session and a new session id (generated with the session_id() function) would appear each time. Checking the session_save_path showed that new sessions were being created each time. In the end, we discovered that IE 7 will not save session cookies if there is an underscore in the domain name. (Our development sites frequently have an underscore in the subdomain name – it’s amazing that we hadn’t found this out earlier.) We’ve replaced the underscore with a hyphen, and everything appears to work correctly.