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session_destroy()
This is only applicable for HTTP initiated scripts. An internal $_SESSION array is available to all HTTP scripts to allow for persistent variables to be shared between scripts. This array is associated with the script based on a cookie created at login.
session_destroy() clears the array. This would clear specific variables like logged_in, an integer indicating whether the user is logged in, so a page in WattmonOS (such as the home page) is called after running this function, you would be redirected to the login page once again.