问在用户登录phpmysql时随时更新上次登录时间

EN

if(isset($_POST['submit'])){
    $username = $_POST['username'];
    $password = $_POST['password'];

    if($user->login($username,$password)){
        $_SESSION['username'] = $username;

        try{
            $stmt = $db->prepare("UPDATE admin  SET login_date = now() where adminID = $adminID ");
            $stmt->execute();
            $stmt=null;
            }
        catch (PDOExeception $e){
                $error[] = $e->getMessage();
            }
        header('Location: index.php');
        exit;
    } 
  else {      
           $error[] =  '<div class="alert alert-danger fade in text-center">
                           <a href="#" class="close" data-dismiss="alert">&times;</a>
                           <strong>Error!</strong> 
                            Wrong username or password or your account has not been activated.
                      </div>';
    }
}