Unable to get session variable to save into my database - php

Question

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\n", $mysqli->error);
}
}
//Get Group info
if(isset($_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\n", $mysqli->error);
}
}
//getting variables and inserting into a database
if($_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

Answer 1

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 -----
}
?>

Answer 2

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