[imp] php session problem

[hui at student.dlut.edu.cn]

    When I login in my imp, I can see 
https://mailb.dlut.edu.cn/horde/imp/mailbox.php?
Horde=1a05c19ca5e8fefe4b7a493f0b26602f&mailbox=INBOX in the url of the ie.
    But if I close the ie without logout. I can still access my mailbox using
the url. I know the reason is the session is still alive in the sever. How can 
I configure the server to make the session dead when user closing the ie?

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