问数据库中的代码不返回

EN

  <?php
class Shortener{
protected $db;

public function __construct(){
    $this->db = new mysqli('localhost','root','wayne123','s');
}
protected function generateCode($num){
    # code...
}
public function makeCode($url)
{
    $url = trim($url);

    if(!filter_var($url, FILTER_VALIDATE_URL)){
        return '';
    }

    $url = $this->db->escape_string($url);

    //Check if exists
    $exists = $this->db->query("SELECT code FROM links WHERE url = '{$url}'");

    if($exists->num_rows){
        return $exists->fetch_objects()->code;
    }
    else{

    }

}
public function getUrl($code){
    # code...
}

  <?php
  session_start();
  require_once 'classes/Shortener.php';

  $s = new Shortener;

  if (isset($_POST['url'])){
$url = $_POST['url'];

if($code = $s->makeCode($url)){
    echo $code;

}else{
    //Problem
}

 }