CHECK PHP SESSION - ISSET($SESSION - IS NOT WORKING [closed]

Question

This is in my page, and it should check that a user is logged in on http://carbonyzed.co.uk/Websites/Jason/sites/2/test/login_success.php

but anybody can assess it, not just those logged in

<?php
session_start();
if( isset($_SESSION["myusername"]) ){
header("location:main_login.html");
}
?>

I have tried

if( isset($_SESSION["myusername"]) ){

and

if( isset($_SESSION[$myusername]) ){

LOGIN CODE perhaps a session isn't being created?

<?php

ob_start();
$host="ClubEvents.db.9606426.hostedresource.com"; // Host name 
$username="ClubEventsRead"; // Mysql username 
$password="Pa55word!"; // Mysql password 
    $db_name="ClubEvents"; // Database name 
$tbl_name="members"; // Table name 

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect"); 
mysql_select_db("$db_name")or die("cannot select DB");

// Define $myusername and $mypassword 
$myusername=$_POST['myusername']; 
$mypassword=$_POST['mypassword']; 

// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes($myusername);
$mypassword = stripslashes($mypassword);
    $myusername = mysql_real_escape_string($myusername);
$mypassword = mysql_real_escape_string($mypassword);
$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);

// If result matched $myusername and $mypassword, table row must be 1 row
if($count==1){

// Register $myusername, $mypassword and redirect to file "login_success.php"
session_register("myusername");
session_register("mypassword"); 
header("location:login_success.php");
}
else {
echo "Wrong Username or Password";
}
ob_end_flush();
?>

Answer 1

Add the NOT operator ! to the if statement. Also don't forget to add exit() after your header. The location header is telling the browser to redirect, but an attacker could view your page and simply ignore the location header, thus bypassing your authentication system becuase your PHP code would continue to execute.

if( !isset($_SESSION["myusername"]) ){
    header("location:main_login.html");
    exit();
}

Furthermore, you aren't calling session_start() in the login code that you posted, therefore the session is not accessible.