session_unregister has been deprecated, this solves the issue.

classes/security_functions.php

@@ -126,11 +126,13 @@ class security_functions
 
 	function closeSale()
 	{
+
 	//deletes sessions vars
-      session_unregister('items_in_sale'); 
+        unset($_SESSION['items_in_sale']);                                      
-    	session_unregister('current_sale_customer_id'); 
+        unset($_SESSION['current_sale_customer_id']);                           
-    	session_unregister('current_item_search'); 
+        unset($_SESSION['current_sale_customer_id']);
-    	session_unregister('current_customer_search'); 
+        unset($_SESSION['current_customer_search']);
+ 
 	}
 
 	function checkMembership($userID)