@PVince81 `i:` means that the next value is an integer, see: http://www.php.net/manual/de/function.serialize.php#66147
Also as you can see you could search in the DB for users that have a notification enabled by ` notify_stream LIKE '%:VALUE_OF_THE_ACTIVITY;%`

Well if we ignore existing activities and just drop them on the update (is that possible?) we could make it use strings instead of integers. But I think that's not what we want to. It would require changing the column in the activities table