created_at 字段是 DATETIME 類型，似乎需要存儲日期時間格式的字符串。

我進行了以下修改：

public function register($username, $password)

{

????......

????$createdAt = date('Y-m-d H:i:s');

????......

}

<?php
/**
* Created by PhpStorm.
* User: lee
* Date: 6/18/2017
* Time: 3:18 PM
*/

require __DIR__.'/lib/User.php';
$pdo=require __DIR__.'/lib/db.php';
$user=new User($pdo);
//

print_r($user->register('admin5','1111'));
echo "<br>";
print_r($user->login('admin5','1111'));
//echo time();

<?php
/**
?*?Created?by?PhpStorm.
?*?User:?lee
?*?Date:?6/18/2017
?*?Time:?3:20?PM
?*/
require?__DIR__.'/ErrorCode.php';
class?User
{
????/**
?????*?數(shù)據(jù)庫鏈接句柄
?????*?@var
?????*/

????private?$_db;

????/**
?????*?構(gòu)造方法
?????*?User?constructor.
?????*?@param?$_db
?????*/


????public?function?__construct($_db)
????{
????????$this->_db=$_db;
????}

????/**
?????*?用戶登陸
?????*?@param?$username
?????*?@param?$password
?????*/
????public?function?login($username,$password)
????{
????????$sql='select?*?from?`user`?where?`username`=:username?and?`password`=:password';
????????$password=$this->_md5($password);
????????$stmt=$this->_db->prepare($sql);
????????$stmt->bindParam(':username',$username);
????????$stmt->bindParam(':password',$password);
????????$stmt->execute();
????????$user=$stmt->fetch(PDO::FETCH_ASSOC);
????????if(empty($user)){
????????????throw?new?Exception('USERNAME_OR_PASSWORD_INVALID',ErrorCode::USERNAME_OR_PASSWORD_INVALID);
????????}
????????unset($user['password']);
????????return?$user;
????}

????/**
?????*?用戶注冊
?????*?@param?$username
?????*?@param?$password
?????*/
????public?function?register($username,$password)
????{
????????if(empty($username)){
????????????throw?new?Exception('USERNAME_CANNOT_EMPTY',ErrorCode::USERNAME_CANNOT_EMPTY);
????????}


????????if(empty($password)){
????????????throw?new?Exception('PASSWORD_CANNOT_EMPTY',ErrorCode::PASSWORD_CANNOT_EMPTY);
????????}

????????if($this->_isUsernameExists($username)){
????????????throw?new?Exception('USERNAME_EXISTS',ErrorCode::USERNAME_EXISTS);
????????}

????????//寫入數(shù)據(jù)庫

????????$sql='INSERT?INTO?`user`?(`username`,`password`,`createdAt`)?values?(:username,:password,:createdAt)';
????????$createdAt=time();//插入mysql變成00000??print_r($stmt);

????????$password=$this->_md5($password);
????????$stmt=$this->_db->prepare($sql);
????????$stmt->bindParam(':username',$username);
????????$stmt->bindParam(':password',$password);
????????$stmt->bindParam(':createdAt',$createdAt);
????????//echo?$createdAt;
????????//print_r($stmt);

????????if(!$stmt->execute()){
????????????throw?new?Exception('fail',ErrorCode::REGISTER_FAIL);
????????}

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


????}

????private?function?_md5($string,$key='imooc')
????{
????????return?md5($string.$key);
????}

????private?function?_isUsernameExists($username)
????{
????????$sql='SELECT?*?from?`user`?where?`username`=:username';
????????$stmt=$this->_db->prepare($sql);
????????$stmt->bindParam(':username',$username);
????????$stmt->execute();
????????$result=$stmt->fetch(PDO::FETCH_ASSOC);
????????return?!empty($result);
????}
}