credo di aver trovato il motivo.

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There's a bug in Internet explorer in which sessions do not work if the name of the server is not a valid name. For example...if your server is called web_server (_ isn't a valid character), if you call a page which uses sessions like http://web_server/example.php your sessions won't work but sessions will work if you call the script like this

[IP NUMBER]/example.php

Took me a lot of time to find out why my PHP sessions worked perfectly in Firefox and Opera but they didn't work in internet explorer

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trovato qui: http://it.php.net/manual/it/ref.session.php#64125