检查PHP会话 - 问题（$ SESSION - 不工作[关闭]

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

<?php

session_start();
if (!isset($_SESSION['myusername']))
{
    header('Location: main_login.html');
}

?>

You need to negate the check.