"；未定义的“；当本地服务器上一切正常时，其他web服务器上出现错误

我已经在本地服务器上运行了很多次这个脚本，没有问题。然而，当我把它们上传到网络服务器上时，我遇到了未定义的错误。我很困惑。

什么问题？？

已定义数据库表。

PHP版本在本地：5.3.5，在web服务器上：5.3.26

我得到这些错误：

            A PHP Error was encountered
            Severity: Notice
            Message:  Undefined index: username
            Filename: models/user.php
            Line Number: 7
            A PHP Error was encountered
            Severity: Notice
            Message:  Undefined index: password
            Filename: models/user.php
            Line Number: 8
            A PHP Error was encountered
            Severity: Warning
            Message:  Cannot modify header information - headers already sent by (output started at /home/caspian/domains/caspiansang.ir/public_html/system/core/Exceptions.php:185)
            Filename: views/login.php
            <p>Line Number: 1

Controller/verifylogin.php:

            <?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
            class VerifyLogin extends CI_Controller {
             function __construct()
             {
               parent::__construct();
               $this->load->model('user','',TRUE);
             }
             function index()
             {
                    $result = $this->user->validate();
                    if(! $result){
                     $data['msg'] = 'اطلاعات وارد شده اشتباه است!';
                    $this->load->view('login',$data);
                    }else{
                        redirect('admin');
                    }        
                }

            }

型号/user.php

            <?php
            Class User extends CI_Model
            {
             public function validate(){
                    $username = $_POST['username'];
                    $password = $_POST['password'];

                    // Prep the query
                    $this->db->where('username', $username);
                    $this->db->where('password', md5($password));
                    // Run the query
                    $query = $this->db->get('admin');
                    // Let's check if there are any results
                    if($query->num_rows() == 1)
                    {
                        // If there is a user, then create session data
                        $row = $query->row();
                        $data = array(
                                'userid' => $row->userid,
                                'fname' => $row->fname,
                                'lname' => $row->lname,
                                'username' => $row->username,
                                'validated' => true
                                );
                        $this->session->set_userdata($data);
                        return true;
                    }
                    return false;
                }

            }

Views/login.php：

       <?php echo form_open('verifylogin'); ?>
           <br>
                    <div style="font-color:black; padding-bottom:10px;"> <?php if(!   empty($msg)) echo $msg;?></div>        
         < label for="username">نام کاربری</label>
         <input type="text" size="20" id="username" name="username"/>
         <br/>
         <label for="password">رمز عبور</label>
         <input type="password" size="20" id="passowrd" name="password"/>
         <br/>