why use magic method __toString() if php return varibale data type?
here is the error message ... must return a string value ... just means the return value of __toString() has to be a value of data type string. If $users_class in your example is not intended to be a string value, it has be to converted to a string before returning it.
But when reading the above example, it seems to me that the var $users_class may just not have been initialized yet. In this case change the method __toString() to :