php – 为什么isset()总是在自定义Object上返回FALSE

<?php

  class User {
    protected $userId; // always filled
    protected $userName; // always filled

    /**
     * Magic method for the getters.
     * 
     * @param type $property
     * @return self|property
     */
    public function __get($property) {
        if (property_exists($this,$property)) {
            return $this->$property;
        } else {
            throw new Exception('property '.$property.' does not exist in '.__CLASS__.' class');
        }
    }

  }

?>

当我从另一个类检查此变量时,使用以下内容：

isset($loggedUser->userName); // loggedUser is my instantiation of the User.php

它返回FALSE ??但是当我在User.php中重载__isset()函数时,我得到了正如我预期的那样：

public function __isset($name)
{
    return isset($this->$name);
}

只是要清楚：

echo $loggedUser->name;   // result "Adis"
isset($loggedUser->name); // results in FALSE,but why?

谢谢你的帮助！