Как правильно убрать подключение с БД?

<?php

class User{
    private $mysqli;

    private $id;
    private $user;
    private $user_id;
    private $is_authorized = false;
    
    public function __construct()
    {
        $this->mysqli = new mysqli("localhost", "root", "12345", "mydb");
    }

    public function __destruct()
    {
        $this->mysqli = null;
    }
    
    public function checkUser($field, $value)
    {
        $user_exist = $this->mysqli->query("SELECT $field FROM users WHERE $field = '$value' LIMIT 1");
        if ($user_exist->num_rows == 0){
            return true;  
        }
        
        return false;
          
    }
    
    
    public function register($data){
        $login = strip_tags(trim($data['login']));
        $email = strip_tags(trim($data['email']));
        $password = strip_tags(trim($data['password']));
        $group = strip_tags(trim($data['group']));

        $valids = $this->validate($login, $email, $password, $group);
        if (empty($valids)){
            $password = md5($password);
            $result = $this->mysqli->query("INSERT INTO users (login, email, password, group_id) VALUES ('$login', '$email', '$password', $group)");
            if ($result){
                header("Location:login.php");
            }
        } else {
            return $valids;
        }
    }


    private function validate($login, $email, $password, $group)
    {
        $errors = array();
        if (strlen($login) < 3 || strlen($login) > 16) {
            $errors['login'] = "Логин должен быть больше 3 символов и не более 16 символов";
        }
        if (!preg_match('~^[a-z0-9_\-]*$~i',$login)) {
            $errors['login'] = "Разрешается только ввод латинских символом, цифр а также знака подчеркивания";
        }

        if (!$this->checkUser('login', $login)){
            $errors['login'] = "Логин $login уже зарегестрирован";
        }

        if (strlen($password) < 3 || strlen($password) > 16) {
            $errors['password'] = "Пароль должен быть больше 3 символов и не более 16 символов";
        }

        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $errors['email'] = "Не верный email адресс";
        }

        if (!$this->checkUser('email', $email)){
            $errors['email'] = "Email $email уже зарегестрирован";
        }
        
        
        if ($group == 0) {
            $errors['group'] = "Выберите группу из списка";
        }

        return $errors;
    }

    public static function isAuthorized()
    {
        if (!empty($_SESSION["user_id"])) {
            return (bool) $_SESSION["user_id"];
        }
        return false;
    }
    
    public static function user(){
        if (!empty($_SESSION["user_id"])) {
            $id = $_SESSION['user_id'];
            //$query = "SELECT id, login, email, group_id, created_at FROM users WHERE id = $id LIMIT 1";
            //$result = $this->mysqli->query($query);
            //return $result->fetch_assoc();
        }
        return false;
    }

    public function authorize($login, $password, $remember = false)
    {
        $password = md5($password);
        $query = "SELECT id, login FROM users WHERE login = '$login' AND password = '$password' limit 1";
        $result = $this->mysqli->query($query);

        $this->user = $result->fetch_assoc();

        if (!$this->user) {
            $this->is_authorized = false;
        } else {
            $this->is_authorized = true;
            $this->user_id = $this->user['id'];
            $this->saveSession($remember);
        }

        return $this->is_authorized;
    }

    public function saveSession($remember = false, $http_only = true, $days = 7)
    {
        $_SESSION["user_id"] = $this->user_id;

        if ($remember) {
            $sid = session_id();
            $expire = time() + $days * 24 * 3600;
            $domain = ""; // default domain
            $secure = false;
            $path = "/";

            $cookie = setcookie("sid", $sid, $expire, $path, $domain, $secure, $http_only);
        }
    }

    public function logout()
    {
        if (!empty($_SESSION["user_id"])) {
            unset($_SESSION["user_id"]);
        }
    }

    public function reset_password($data){
        $email = strip_tags(trim($data['email']));
        if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $check = $this->checkUser('email',$email);
            if (!$check) {
                $password = $this->randomString();
                $pass = md5($password);
                $this->mysqli->query("UPDATE users SET password = '$pass' WHERE email = '$email'");
                $this->sendMail($email, $password);
                return true;
            }
        }else {
            $check = $this->checkUser('login',$email);
            if (!$check) {
                $password = $this->randomString();
                $pass = md5($password);
                $result = $this->mysqli->query("UPDATE users SET password = '$pass' WHERE login = '$email'");
                $user = $result->fetch_assoc();
                $this->sendMail($user['email'], $password);
                return true;
            }
        }

        return false;

    }

    private  function randomString($length = 10) {
        return substr(str_shuffle(str_repeat($x='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', ceil($length/strlen($x)) )),1,$length);
    }

    private function sendMail($email, $password){
        $title = "Восстановления пароля";
        $headers  = 'MIME-Version: 1.0' . "\r\n";
        $headers .= 'Content-type: text/html; Charset=utf-8' . "\r\n";
        $msg = 'Ваш новый пароль:'.$password;
        mail($email, $title, $msg, $headers);
    }

}

$user = new User();

?>