Check whether the logon status is correct.

Determine whether the logon status is in. for example, when I log on, I execute the sqlupdateusersetislogin = '1' where condition, but how can I set it to zero when I exit? If you click the exit button, you can still use SQL for update. However, if you do not click the exit button, you only need to check whether the account is logged on.
For example, when I log in
Execute SQL update user set islogin = '1' where condition

But how can I set it to zero when I exit?
If you click the exit button, you can still use SQL to update

However, if you do not click the exit button, but the session storage does not exist,
What should I do?

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Cookie seesion sets the expiration time, because, for example, when the browser crashes, or is not properly closed, you cannot set the field in the database to 0.
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Generally, session processing is used for this status. when the browser is closed, the "logon status" is automatically cleared.
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Click exit.
Do not click the exit button, but the session does not exist:
You need to synchronize the session and database content. when you log on, you can determine that if this field in the session is 0, but this attribute field is 1 in the background database, the database field should be synchronized to 0, requiring the user to enter the user name and password to log on again

Session can also prevent at the same time with an account to log on, but is to squeeze the previous user, the specific method can see: http://www.cnblogs.com/heaiping/archive/2010/01/19/1651835.html
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When exiting, pass a GET parameter

Then the GET parameter exists.
Execute the UPDATE statement + session_destroy ()
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We recommend that you check the session mechanism.