PHP PDO OOP Call to a member function prepare

PHP kode 

 class DB {
     private static $_instance = null;
     private $_query, $_error, $_pdo;
     private $user = "root", $pass = "";
     
     private function __construct() {
         try {
             $this->_pdo = new PDO('mysql:host=127.0.0.1;', 'dbname=zinxan', $this->user, $this->pass);
         } catch (PDOException $e) {
             die($e->getMessage());
         }
     }
     
     public static function getInstance() {
         if (!isset(self::$_instance)) {
             self::$_instance = new DB();
         }
         return self::$_instance;
     }
     
     public function Query($sql, $params = array()) {
 
         $this->_error = false;
         
         if ($this->_query = $pdo->prepare($sql)) {
             $x = 1;
             if (count($params)) {
                 foreach ($params as $param) {
                     $this->_query->BindValue($x, $param);
                     $x++;
                 }
             }
             
             if($this->_query->execute()) {
                 echo 'Success';
             }
         }
     }
 }

PHP kode 

 $x = "Mathias";
 $user = DB::getInstance()->Query("SELECT username FROM users WHERE username = ?", array($x));
 if ($user->error()) {
     echo 'No user';
 } else {
     echo 'User found';
 }