在PHP MYSQL中将表单数据从一个页面传输到许多其他页面

我有一个问题，我想将表单数据从"home.php"传输到"dashboard.php"，但这里我在dashboardphp上检查了登录会话的会话，如下所示：-

    <?php  
session_start();
?>
<?php
if(!$_SESSION["UserName"])
{
    //Do not show protected data, redirect to login...
    header('Location: login.php');  
}
?>

在这里，当用户从"home.php"提交表单数据时，如果没有生成登录会话，他会被重定向到登录页面进行登录。在主页上，我使用POST方法形成如下：-

 <form class="form ajax-contact-form" method="POST" action="dashboard.php">
                            <div class="alert alert-success hidden" id="contact-success">
                                <span class="glyphicon glyphicon-ok "></span> &nbsp;
                                <strong>Success!</strong> Thank you for your message.
                            </div>
                            <div class="alert alert-danger hidden" id="contact-error">
                                <span class="glyphicon glyphicon-remove "></span> &nbsp;
                                <strong>Error!</strong> Oops, something went wrong.
                            </div>
                            <div class="row col-p10">
                                <div class="col-sm-6">
                                    <label class="mb10">
                                        <input type="text" name="name_" id="name_" required class="form-control" placeholder=" Full Name * ">

                                    </label>
                                </div>
                                <div class="col-sm-6">
                                    <label class="mb10">
                                        <input type="text" name="subject_" id="subject_" required class="form-control" placeholder=" Subject *">
                                    </label>
                                </div>
                            </div>
                            <div class="row col-p10">
                                <div class="col-sm-6">
                                    <label class="mb10">
                                        <input type="text" name="phone_" id="phone_" class="form-control" placeholder=" Phone">
                                    </label>
                                </div>
                                <div class="col-sm-6">
                                    <label class="mb10">
                                        <input type="email" name="email_" id="email_" required class="form-control" placeholder=" Email Address *">
                                    </label>
                                </div>
                            </div>
                            <label>
                                <textarea name="message_" id="message_" cols="30" rows="10" required class="form-control" placeholder=" Message *"></textarea>
                            </label>
                            <div class="mb40"></div>
                            <div class="clearfix">
                            <!-- Enter your google site key here for captcha -->
                               <div class="pull-right xs-pull-left xs-box">

                                </div>  
                                <div class="pull-left">
                                    <button type="submit" class="btn btn-icon btn-e" value="submit" name="submit"><i class="icon icon_mail_alt"></i> Sumbit</button>
                                    <?php 
                                             if (isset($_POST['submit'])) { 
                                             $_SESSION['name_'] = $_POST['name_'];
                                             $_SESSION['subject_'] = $_POST['subject_'];
                                             } 
                                    ?> 
                                </div>
                            </div>
                        </form>

登录页面上的代码：-

  <?php  
include("connection.php");
session_start();//session starts here  
?>  
<?php
if(isset($_SESSION["UserName"]))
{
    //Do not show protected data, redirect to login...
    header('Location: dashboard.php');
}
?>
<?php   
                                    if(isset($_POST['login']))  
                                    {  
                                        $username=$_POST['Username'];  
                                        $user_pass=$_POST['password'];  
                                        $encrypt_pass = md5($user_pass);

                                        $check_user="select * from tbl_logindetails WHERE UserName='".$username."' AND Password='".$encrypt_pass."'";

                                        $run=mysqli_query($connection,$check_user); 
                                        if(mysqli_num_rows($run))  
                                        {  
                                            //echo "<script>window.open('www.google.com','_self')</script>"; 
                                            header("Location: dashboard.php");                                  
                                            $_SESSION['UserName']= $username; //here session is used and value of $user_email store in $_SESSION.  

                                        }  
                                        else  
                                        {  
                                          echo "<script>alert( 'Error in Registering Useer, Please try again later' )</script>";  
                                        }  
                                    }  
                                    ?> 

显示"home.php"到"dashboard.php"数据的代码

                   <?php echo $_POST["name_"]; ?>
                    </br> </br>
                    <?php echo $_POST["subject_"]; ?> 
                    </br> </br>
                    <?php echo $_POST["message_"]; ?> 
                    </br> </br>

登录后，用户可以访问dashboard.php，但我的问题是，当用户登录后，我从"home.php"中获取的输入数据显示了一个错误，如下所示：附上图像链接：-

[1]: https://i.stack.imgur.com/G5Udj.png

我在完成以上表单数据处理后收到的错误-->如果没有会话则登录-->Dashboard

注意：第401行C:''examplep''htdocs''youngants''dashboard.php中的未定义索引：name_

注意：未定义的索引：subject_ in C:''examplep''htdocs''youngants''dashboard.php，位于第403行

注意：未定义的索引：message_在C:''examplep''htdocs''youngants''dashboard.php中的第405行

注意：第408行C:''examplep''htdocs''youngants''dashboard.php中的未定义索引：name

伙计们，我需要你们的帮助，我如何通过会话变量将数据从"home.php"携带到"dashboard.php"，甚至可以在用户提交表单后重定向到登录，然后在登录后返回dashboard.php后保持填充状态。

任何需要的更多信息，请评论，我会提供它。