function _mySite_session($key, $value = NULL)
{
    static $storage;
    if ($value)
    {
        $storage[$key] = $value ;
        $_SESSION['mykey'][$key] = $value ; // I use 'mykey' as a session key because in case some other module uses 'type' in $_SESSION. So try to make it unique.
    }
    elseif (empty($storage[$key]) && isset($_SESSION['mykey'][$key]))
    {
        $storage[$key] = $_SESSION['mykey'][$key];
    }
    return $storage[$key];
}

_mySite_session("USERNAME", "Mr. John");

$username = _mySite_session("USERNAME");