PHP Sessions: Explanation please? - php

Question

I have been learning PHP for a little bit now, and it has been going really easy for the most part. The only thing I'm hung up on is getting sessions to work. Google has been unforgiving in this endeavor.
It could be one of two reasons; syntax or my software. I'm currently building a local website using EasyPHP 5.3.5.0 on a machine that isn't connected to the internet. Connecting it to the internet is not an option.
What I currently know of sessions is that a lot of syntax related to it has be deprecated, replaced by the superglobal $_SESSION array, which is a lot easier to use. start_session(); must be before any syntax relating to sessions. However, my login script isn't establishing a session, as a quick !isset ($_SESSION['username']) always returns true.
My script is set up like this:
PHP include to login.php, which is a form. check_login.php is what validates it, and if a query returns one row, it'll redirect to login_success.php which establishes the session, gives a welcome message then redirects (Using JavaScript) to the homepage.
Any ideas?
EDIT to include more information:
Here is a synopsis of my code:
index.php:
include 'main_login.php';
main_login.php:
if(!isset ($_SESSION['username'])){
...
Login form, action="cehcklogin.php" method="post"
...
}else{
var_dump ($_SESSION): // Just to see if it works
}
checklogin.php:
Connect to SQL
$username = $_POST['username'];
$password = $_POST['password'];
$username / $password stripslashes / mysql_real_escape_string
Query to find the username & password
$count = mysql_num_rows($result);
if($count = 1){
$_SESSION["username"] = $username;
$_SESSION["password"] = $password;
header("location:login_success.php");
}else{
echo "Wrong Username or Password."
}
login_success.php:
The login process goes to all of the way here, redirects home and that's where the problem is.
session_start();
var_dump($_SESSION); //This works
if(!isset ($_SESSION['username'])){
header("location:index.php");
}
Javascript redirect, and a welcome message appears.
It all works until you get to the homepage, which $_SESSION['username'] should be set, and it should not display the form, but it does.

Answer 1

It looks like you're not using session_start() in your main_login.php like etranger alluded to. You need to call that function at the start of each new request to begin using sessions.
Otherwise, if you are calling session_start() and you just neglected to show it in the code sample, then maybe the session ID is being lost during the redirect. Are you using cookie-based sessions or passing session ID as a URL parameter? Try printing session_id() or SID at the top of each page. This will let you know when the session is lost (the session ID will change or be "").
If you're using cookie-based sessions, then maybe the cookie is getting lost for some reason. If you're using URL parameter to pass session ID, then maybe transparent session ID support isn't working right.

Answer 2

You have to call session_start() as early as possible, and definitely before using $_SESSION, which would otherwise be empty.