SessionUnSet
Destroys the named session variable.
The framework tracks only 2 session variables. These are UID, which
is user_id, and PWD, which is user password. An application should
never call SessionUnSet on these variables.
string $key - variable to destroy
function SessionUnSet($key,$context='app',$sfx='app') {
$x=$context;
$xkey=$GLOBALS["AG"]["application"]."_".$sfx."_".$key;
unset($_SESSION[$xkey]);
}