phone verification module

[mojtaba sabet]

I write a costume phone verification module for WHMCS

and i used the following hook to redirect clients to verification module before they check out their order.

   add_hook('ShoppingCartValidateCheckout', 1, "PV");

   function PV($vars) {
   $clientID = intval($_SESSION['uid']);
   $isVerified = //check from database
   if ($isVerified != 'true') {

            return '<a href="index.php?m=module">send code</a>';
        } 

this hook works fine when client is logged in but for new clients that register in page cart.php?a=view this hook not work because $_SESSION['uid'] returns 0 . which hook should i use for the above exception?