用于数据库访问的PHP对象似乎与mysqli

Question

这是我的DatabaseAccessor类：

<?php
    class DatabaseAccessor 
    {
        public $db;

        public static function set_db($new_db){
           $this->$db = $new_db;
        }

        public function modifyDatabase($query){
            $db->query($query);
        }

        public function readFromDatabase($query){
            $temp = $db->query($query);
            return $temp;
        }
    }
?>

出于某种原因，当我尝试在另一个文件中使用它时，它不会工作。这是我尝试使用它的文件：

<?php
include("/Back-End/DatabaseAccessor.php");
session_start();

$DataBaseAccessor = new DatabaseAccessor();
$temp = new mysqli(getenv('IP'), getenv('C9_USER'), "", "Moneybags", 3306);
$DataBaseAccessor->set_db($temp);

$desiredName = $_POST["username"];
$desiredPassword = $_POST["password"];
$desiredGender = $_POST["gender"];

$query = "INSERT INTO tblUsers (username, password, gender) VALUES ('" . $desiredName . "', '" . $desiredPassword . "', '" . $desiredGender . "')";

try {
    $DataBaseAccessor->modifyDatabase($query);
    $query = "SELECT * FROM tblUsers WHERE username = '" . $desiredName . "' AND password = '" . $desiredPassword . "'";
    $res = $DataBaseAccessor->ReadFromDatabase($query);
    $res->data_seek(0);
    $row = $res->fetch_assoc();

    $_SESSION["LoggedIn"] = true;
    $_SESSION["UserID"] = $row['UserID'];
    $_SESSION["Username"] = $row['username'];
    header("refresh:7;url=index.php");
    echo "Account created!";
    echo "Returning to main page momentarily...";

} 
catch (Exception $e){
  echo "Something went wrong with the account creation...";
  echo "Returning to main page temporarily...";
  header("refresh:7;url=index.php"); 
}
?>

我尝试将参数传递给DatabaseAccessor对象，并在其中使用构造函数将mysqli参数设置为公共属性。那不行。我知道我的查询有效，因为当我在每个需要访问数据库的类中创建新的mysqli对象时，它们工作正常。