error related to session_start() or not?

Question

I have a page on my site that I am making that is to upload comments to a community section from the users profile page.

The page was working fine but I needed to add a 'username' foreign key to the table to identify who made the post, and so I am trying to use the session function to do this.

In the code below, the three functions I added (before which the page was working correctly) are the $date, $time, and $_SESSION['username']=$_POST['username'].

The $date and $time variables are for timestamping the post, and along with the $_SESSION['username']=$_POST['username'], are going to be the primary key for the table.

Since I added those three variables, (yes, they are added in the table) I am getting an error:

Notice: Undefined index: username in C:\xampp\htdocs\soiree'_2\create_post_script.php on line 12

Notice: Undefined index: username in C:\xampp\htdocs\soiree'_2\create_post_script.php on line 22 failure

It seems to have a problem with the $_SESSION variables. So with that being said, is my implementation of the session variable incorrect?

As you can see below the page begins with a 'start_session();' function, so does my log in script. The way I understand this to work is when my user logs on, the $_POST['username'] is assigned to $_SESSION['username'], so that on the subsequent pages, like when they go to make their post or comment, $_SESSION['username'] captures the credentials to insert into the column of the table for 'username'.

Perhaps obvious this is my first time using session variables/functions. Below is the code for the page in question (the comment page) and the script for the login below. Thanks.

    <?php
    session_start();

    include('db.php');


     $event_name='';
     $place='';
      $t='';
      $d='';
     $description=''; 
     $_SESSION['username'];
     $time='';
     $date='';


    $event_name=strip_tags($_POST['event_name']);
    $place=strip_tags($_POST['place']);
    $t=strip_tags($_POST['time']);
    $d=strip_tags($_POST['date']);
    $description=strip_tags($_POST['event_description']);
    $_SESSION['username']=strip_tags($_POST['username']);
    $time=time();
     $date=date("y-m-d");

   if(isset($_POST['submit'])) 
    {            
    $query = "INSERT INTO user_posts (date, time, username, title, location, t, d, description)"; 
    $query .= "VALUES ('$date','$time','$_SESSION["username"]','$event_name','$place','$t','$d','$description')";

    if (mysqli_query($connection, $query)) {
        echo "<h2> your post has been submitted </h2>";        
        }        
    else {            
        die('failure');            
    }     
}

and the following is from the login script;

        <?php 
         session_start();
          include("db.php");
          if(isset($_POST['submit'])) {

          $username = $_POST["username"];
          $password = $_POST["password"];

         $u_check= mysqli_query($connection, "SELECT username AND password   FROM registration WHERE username='$username' AND password='$password'");
          $check=mysqli_num_rows($u_check);

         if($check==1) {

          $_SESSION['auth'] = "yes";
         $_SESSION['username'] = $_POST['username'];
        header("location: home.php");//echo "succesful login";

         }
        else {
        echo "invalid login info";        
        $result=mysqli_query($connection, $query);

        if(!$result){    
        die('failure');
        }
        }
        }