public function getUsers($user_id)
 {
     if (empty(self::$conn)) {
         self::$conn = $this->connect_pdo();
     }
     $sql = "SELECT username,CONCAT_WS(' ',firstname,lastname) AS name,email FROM users WHERE user_id=?";
     $stmt = self::$conn->prepare($sql);
     $stmt->bindParam(1, $user_id);
     $stmt->execute();
     $stmt->setFetchMode(PDO::FETCH_ASSOC);
     $users = array(array());
     $i = 0;
     while ($row = $stmt->fetch()) {
         $users[$i++] = $row;
     }
     $i--;
     return $users;
 }