将重复条目捕获在寄存器数据中

Question

我有一个注册表格，然后这将在我以表格注册帐户后阅读的内容，

   $('#manage-user').submit(function(e){
            e.preventDefault();
            start_loader()
            $.ajax({
                url:_base_url_+'classes/Users.php?f=register',
                data: new FormData($(this)[0]),
                cache: false,
                contentType: false,
                processData: false,
                method: 'POST',
                type: 'POST',
                success:function(resp){
                    if(resp ==1){
              alert('Registration Successful. You may now log-in.')
                        window.location.reload()
                    }else{
                        $('#msg').html('<div class="alert alert-danger">Email already exist</div>')
                        end_loader()
                    }
                }
            })
        })

---

这是我的功能。

public function register(){
        extract($_POST);
        $data = '';
        foreach($_POST as $k => $v){
            if(!in_array($k,array('id','password'))){
                if(!empty($data)) $data .=" , ";
                $data .= " {$k} = '{$v}' ";
            }
        }


    
        
        if(!empty($password)){
            $password = md5($password);
            if(!empty($data)) $data .=" , ";
            $data .= " `password` = '{$password}' ";
        }

        if(isset($_FILES['img']) && $_FILES['img']['tmp_name'] != ''){
                $fname = 'uploads/'.strtotime(date('y-m-d H:i')).'_'.$_FILES['img']['name'];
                $move = move_uploaded_file($_FILES['img']['tmp_name'],'../'. $fname);
                if($move){
                    $data .=" , avatar = '{$fname}' ";
                    if(isset($_SESSION['userdata']['avatar']) && is_file('../'.$_SESSION['userdata']['avatar']))
                        unlink('../'.$_SESSION['userdata']['avatar']);
                }
        }
        if(empty($id)){
            $data .= " ,`type` = '2' ";
            $qry = $this->conn->query("INSERT INTO users set {$data}");
            if($qry){
                return 1;
            }else{
                return 2;
            }

        }else{
            $qry = $this->conn->query("UPDATE users set $data where id = {$id}");
            if($qry){
                $this->settings->set_flashdata('success','User Details successfully updated.');
                foreach($_POST as $k => $v){
                    if($k != 'id'){
                        if(!empty($data)) $data .=" , ";
                        $this->settings->set_userdata($k,$v);
                    }
                }
                if(isset($fname) && isset($move))
                $this->settings->set_userdata('avatar',$fname);

                return 1;
            }else{
                return "UPDATE users set $data where id = {$id}";
            }
            
        }
    }

---

当我添加此行时，

        $user_check_query = "SELECT * FROM users WHERE username='$username' LIMIT 1";
        $result = mysqli_query($db, $user_check_query);
        $user = mysqli_fetch_assoc($result);

它促使电子邮件已经存在，但仍创建一个帐户，我需要添加或更改什么？ 我是编码的初学者，所以我真的不知道该怎么办..

任何人都可以帮我谢谢