Caught exception: 注册失败 Array ( [userId] => 0 [username] => admin1 [createAt] => 1502333216 ),老师我这个问题什么情况,我的userId设置的是自增的

我的代码

user.php

public function register($username,$password)
   {
       if (empty($username)) {
           throw new Exception('用户名不能为空', ErrorCode::USERNAME_CANNOT_EMPTY);
       }
       if (empty($password)) {
           throw new Exception('密码不能为空', ErrorCode::PASSWORD_CANNOT_EMPTY);
       }
       if ($this->_isUsernameExists($username)) {
           throw new Exception('用户名已存在', ErrorCode::USERNAME_EXISTS);
       }
       //写入数据库
       $sql = "INSERT INTO 'user'('username','password','createdAt') VALUES (:username,:password,:createdAt)";
       $createdAt = time();
       $password = $this->_md5($password);
       $stmt = $this->_db->prepare($sql);
       $stmt->bindParam(':username', $username);
       $stmt->bindParam(':password', $password);
       $stmt->bindParam(':createdAt', $createdAt);
//        print_r($stmt);exit();

       if (!$stmt->execute()) {
           try{
           throw new Exception('注册失败', ErrorCode::REGISTER_FAIL);
           }catch (Exception $e) {
               echo 'Caught exception: ',  $e->getMessage(),'<br>';
           }
       }

       return[
       'userId' => $this->_db->lastInsertId(),
       'username' => $username,
       'createAt' => $createdAt
       ];
   }

index.php

require_once __DIR__.'/lib/User.php';
$pdo = require __DIR__.'/lib/db.php';
$user=new User($pdo);
//print_r( $pdo -> errorInfo(), true);
print_r($user->register('admin1','admin1s'));