Allow content with $_Session - php

Question

I've been following some tutorials and managed to get my login and logout scripts working. What I"m now trying to do it get it to only allow access to pages when the user is logged in. Right now it's just redirecting users to the login page every time, which tells me that the session isn't being set or or my code is just wrong (and I've tried everything I can think of)
This is the login.php script that my form runs in order to set the session:
<?php
// establishing the MySQLi connection
require 'init.php';
if (mysqli_connect_errno())
{
echo "MySQLi Connection was not established: " . mysqli_connect_error();
}
// checking the user
if(isset($_POST['login'])) {
$username = mysqli_real_escape_string($conn,$_POST['username']);
$pass = mysqli_real_escape_string($conn,$_POST['password']);
$sel_user = "select * from login where username='$username' AND password='$pass'";
$run_user = mysqli_query($conn, $sel_user);
$check_user = mysqli_num_rows($run_user);
if($check_user>0) {
$_SESSION['username']=$username;
echo "<script>window.open('index.php','_self')</script>";
} else {
echo "<script>alert('Sorry. Your username or password is not correct, try again!')</script>";
}
}
?>
And this is what I'm including at the top of every page:
<?php
session_start();
if (!(isset($_SESSION['username']) && $_SESSION['username'] != '')) {
header ("Location: account-login.php");
}
require 'init.php';
?>
I switched the login.php file from directing to a page to a popup telling me that I logged in and I get the popup, so the user and password are registering fine, it's just not storing the session somehow. Any ideas? Thanks!

Answer 1

OK, so I got it to work finally!
Apart from all the comments (which helped a TON), I also decided to change the name I was setting in $_SESSION. I think it may be because the session name matched the name or POST data and that eas causing a conflict somewhere.
Changed this:
$_SESSION['username']=$username;
Which I think conflicted to this:
$_SESSION['session_id']=$username;
Worked!
THANK YOU!!!!!!!