Allowing different types of arguments to functions (e.g. int|array) make them unnecessarily complex. Instead, two functions/methods should be used that have clearly defined parameters.

Also, some of the recursive calls do not seem to make any sense at all.

	public function delete_notifications($item_type, $item_id)

	{

		if (is_array($item_type))

		{

			foreach ($item_type as $type)

			{

				$this->delete_notifications($type, $item_id);

			}

			return;

		}

Why would one want to delete topic_id 4242123 and post_id 4242123 at the same time? They have nothing to do with each other.