在子类中调用父构造函数时出错

Question

我想在子类中调用父构造函数。这是System.php文件中的父类：

class System
{
    private $db_host;
    private $db_name;
    private $db_user;
    private $db_pass;
    private $connection;
    protected $glb;

    public function __construct()
    {
        $this->db_host = "localhost";
        $this->db_name = "signage";
        $this->db_user = "root";
        $this->db_pass = "";
        $this->glb =& $GLOBALS;
    }
}

以及User.php文件中的子类：

require_once "System.php";

class User extends System
{
    public $user_id;
    public $user_fullname;
    public $user_password;
    public $user_email;
    public $user_token;
    public $user_type;

    public function __construct()
    {
        parent::__construct();
    }
}

返回此错误：

Fatal error: Uncaught Error: Cannot call constructor in C:\xampp\htdocs\signage\classes\User.php:14 Stack trace: #0 C:\xampp\htdocs\signage\login.php(6): User->__construct() #1 {main} thrown in C:\xampp\htdocs\signage\classes\User.php on line 14

问题出在哪里？我见过constructors and distructors in php manual，那里一切正常。