致命錯誤：未捕獲錯誤：在 X:\x\x\x\class\auth.php:29 中的 null 上調用成員函數prepare() 堆棧跟蹤：#0 x:\x\x\index.php(8 ): Auth->register_user() #1 {main} 在第 29 行的 x:\x\x\x\class\auth.php 中拋出這是數據庫連接文件class getDB {    protected $conn;    public $db_host = 'localhost';    public $db_user = 'root';    public $db_pass = '';    public $db_name = 'f_base';    public function getConn() {        try {            $this->conn = new PDO("mysql:host=$this->db_host;dbname=$this->db_name", $this->db_user, $this->db_pass);            $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);            $this->conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);        } catch ( PDOException $e ) {            die ('<h1>ERROR:</h1><h2>'.$e->getMessage().'</h2>');        }    }    // TODO: Dodati funkcije koje ce moci da ucitaju databazu da bi se prikazalo nesto iz databaze }該類用于注冊class Auth {    public function __construct () {        $connection = new getDB();        $this->conn = $connection->getConn();        return $this->conn;    }    public function register_user ( $username, $email, $password, $r_date ) {        $reg_user = $this->conn->prepare("INSERT INTO korisnici ( username, email, password, r_date ) VALUES ( ?, ?, ?, ? )"); // THIS IS LINE I AM GETTING ERROR        $reg_user->execute( array( $username, $email, $password, $r_date ) );            }}我收到錯誤的這一行 $reg_user = $this->conn->prepare("INSERT INTO korisnici ( username, email, password, r_date ) VALUES ( ?, ?, ?, ? )"); // THIS IS LINE I AM GETTING ERROR