ipaylinks HTTP_REFERER notice

webht/third_party/pay/controllers/iPayLinksService.php

@@ -697,7 +697,7 @@ class IPayLinksService extends CI_Controller
             $ret->data = $respObject = (Object) $resp;
             $ref = "";
         } else {
-            $ref = $_SERVER['HTTP_REFERER'] ? $_SERVER['HTTP_REFERER'] : NULL;
+            $ref = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : NULL;
             $r = iconv("UTF-8", "UTF-8//IGNORE", $resp);
             $ret->data = $respObject = @ simplexml_load_string($resp);
             // test