无法获取会话变量以保存到我的数据库中

Can anyone help me in getting the group_id from a session and save into the database, it seems not to working, been working on it for a while now. The error i am getting is Notice: Undefined index: group_name This is my script

include('db.php')  

//Get User Info

    if(isset($_SESSION['username'])){
    $LoggedUser = $_SESSION['username'];
    if($GetUser = $mysqli->query("SELECT * FROM users WHERE    username='$LoggedUser'")){
    $UserInfo = mysqli_fetch_array($GetUser);
    $LoggedUsername = strtolower($UserInfo['username']);
    $LoggedUserLink   = preg_replace("![^a-z0-9]+!i", "-", $LoggedUsername);
    $LoggedUserLink   = strtolower($LoggedUserLink);
    $UserId = $UserInfo['user_id'];
    $GetUser->close();

}else{

     printf("Error: %s
", $mysqli->error);

}
}
//Get Group info
if(isset($_SESSION['group_name'])){
$LoggedGroup = $_SESSION['group_name'];
if($GetGroup = $mysqli->query("SELECT * FROM groups WHERE group_name='$LoggedGroup'")){
    $GroupInfo = mysqli_fetch_array($GetGroup);
    $LoggedGroupname = strtolower($GroupInfo['group_name']);
    $LoggedGroupLink   = preg_replace("![^a-z0-9]+!i", "-", $LoggedGroupname);
    $LoggedGroupLink      = strtolower($LoggedGroupLink);
    $GroupId = $GroupInfo['group_id'];
    $GetGroup->close();
}else{
     printf("Error: %s
", $mysqli->error);
}
   }

//getting variables and inserting into a database

if($_POST)
{   
    $User     = $UserId;
    $Group    = $GroupId;
    $mysqli->query("INSERT INTO tb_name(group_id_fk, user_id_fk) VALUES ('$Group', '$User')");

     die('<div class="alert alert-success" role="alert">You have been added successfully to the group.</div>');
}else{
    die (mysqli_error());
}
?>

Thanks

A session is started with the session_start() function.

Session variables are set with the PHP global variable: $_SESSION.

<?php
// Start the session
session_start();

// Set session variables
 if(isset($_SESSION['username'])) {

    ---- Your Statements -----

 }
?>

you need to start a session first by session_start(); before using $_SESSION global