Logging in not creating a session - php

Question

I'm trying to log in to my site, I'm looking at the database and the username/password are correct. When logging in I should be greeted by ja message that says "Welcome Ryemck" as "Ryemck" is my username, but this could be replaced by any username that is logged in. Here's the code to show that:
<td>Welcome </td>
<td><i><?php echo $username ['username']; ?></i></td>
However, instead of that nice message I get "Notice: Undefined variable: username ", I assume this is because the username isn't being set as the variable as it should in this code.
$username=$_POST['username'];
I also have this code that SHOULD give me the error if no username/password are entered, but it doesn't, so this doesn't work. is "session_start();" not the correct session code?
if (isset($_POST['submit'])) {
if (empty($_POST['username']) || empty($_POST['password'])) {
$error = "Username or Password is invalid";
EDIT
<?php
session_start(); // Starting Session
echo "hello";
$error=''; // Variable To Store Error Message
if (isset($_POST['submit'])) {
if (empty($_POST['username']) || empty($_POST['password'])) {
$error = "Username or Password is invalid";
}
else
{
echo "hello";
// Define $username and $password
$username=$_POST['username'];
$password=$_POST['password'];
// To protect MySQL injection for Security purpose
$username = stripslashes($username);
$password = stripslashes($password);
$username = mysqli_real_escape_string($username);
$password = mysqli_real_escape_string($password);
// Establishing Connection with Server by passing server_name, user_id and password as a parameter
$connection = mysqli_connect("localhost", "root", "");
// Selecting Database
$db = mysqli_select_db("game", $connection);
// SQL query to fetch information of registerd users and finds user match.
$query = mysqli_query("select * from login where password='$password' AND username='$username'", $connection);
$rows = mysqli_num_rows($query);
if ($rows == 1) {
$_SESSION['login_user']=$username; // Initializing Session
header("location: header.php"); // Redirecting To Other Page
} else {
$error = "Username or Password is invalid";
}
mysqli_close($connection); // Closing Connection
}
}
?>

Answer 1

Change it to:
<td><i><?php echo $username; ?></i></td>

Answer 2

$username is not an array , gli must write :
<td>Welcome </td>
<td><i><?php echo $username ; ?></i></td>

Answer 3

I am not sure if i am reading this right are you looking to start a new session at the top of the page and the store values in that session?.
if you place something like the following at the top of your page it will remember sessions variables
if(!isset($_SESSION)){session_start());
You would then assign to a session variable using
$_SESSION["userName"] = $_POST["userName"];
To tell if its be set to grant access to a admin console you could do something like
if(isset($_SESSION["userName"]) && $_SESSION["userName"] != '')
the above means that the variable has been set but you would probably wrap the session userName in a db function to check for valid logins.

Answer 4

The button I was clicking was set to "login" whereas the isset was set to "submit".
Simply changing the words solved it.