Cool, that is great news to hear. About setting the default value of the session variable, this is one way to get past the warning, another way of doing this is to check that the variable is set before you try to make use of it. So in your if statement where you are checking on the value of the session variable you can use an isset like this:
if (isset($_SESSION['DL_ATC_ID']) && $_SESSION['DL_ATC_ID'] == $row_rsBooks["id_inventory"]){