function fetch_callback($sql, callable $callback)
	{
		$result = null;
		$rs = $this->query($sql, MYSQLI_USE_RESULT);
		while ($row = $rs->fetch_assoc())
		{
		//	if (call_user_func_array($callback, [&$row, &$result]) === false)	//	alternative: when using references, the values MUST be passed via an array with call_user_func_array(), call_user_func() only passes by value!
			if ($callback($row, $result) === false)
				break;
		}
		$rs->free_result();
		return $result;
	}