Logout script is not working - php

Question

Please help with my logout script. I am sorry for dumb mistakes I am very new to php. Please provide me with details and examples of how to fix this. Thank you so much.
Login Page: There are 4 types of users. Each user will get a separate home page.
<?php
session_start();
require_once('common/config.php');
if(isset($_POST['username']))
{
$username = $_POST['username'];
$password = $_POST['password'];
$sql = "SELECT * FROM `users` WHERE username='".$username."' and password='".$password."'";
$result = mysql_query($sql) or die(mysql_error());
$fetched = mysql_fetch_array($result);
if ($fetched['user_type'] == "admin")
{
header('location: adminhomepage.php');
}
else if ($fetched['user_type'] == "po")
{
header('location: pohomepage.php');
}
else if ($fetched['user_type'] == "pw")
{
header('location: pwhomepage.php');
}
else if ($fetched['user_type'] == "ps")
{
header('location: pshomepage.php');
}
else
{
header('location: invalid.php');
exit();
}
}
?>
Home Page: For instance this is the admin home page.
<?php
session_start();
if (isset($_SESSION['username']) && ($_SESSION['username'] !== 1))
{
header('Location: login.php');
}
?>
Logout Page
<?php
session_start();
$_SESSION['username'] =0;
?>
Logout Button
<form action = "logout.php">
<input id="logoutbutton" type="submit" value="Logout">
</form>

Answer 1

logout page
<?php
session_start();
unset($_SESSION['susername']);
$_SESSION['susername'] = "";
session_destroy();
header("location:index.php");
?>
login.php
session_start();
if (isset($_SESSION['uname']) == "")
{
require_once('index.php');
}
$user_name = $_POST['user_name'];
$user_pass = $_POST['user_pass'];
$_SESSION['susername'] = $user_name; // or other value

Answer 2

logout page -
<?php
session_start();
$_SESSION['username'] =0;
?>
you have to start the session first before accessing it.

Answer 3

In your logout.php please update this code...
<?php
session_start();
if(isset($_SESSION['username']))
{
unset($_SESSION['username']);
header('Location: login.php');
}
?>

Answer 4

use unset($_SESSION['susername']) or session_destroy

Answer 5

Edit:
With your new information, it's clear you never actually set the $_SESSION['username'] value.
if ($fetched['user_type'] == "admin")
{
$_SESSION['username'] = $username;
header('location: adminhomepage.php');
}
else if ($fetched['user_type'] == "po")
{
$_SESSION['username'] = $username;
header('location: pohomepage.php');
}
else if ($fetched['user_type'] == "pw")
{
$_SESSION['username'] = $username;
header('location: pwhomepage.php');
}
else if ($fetched['user_type'] == "ps")
{
$_SESSION['username'] = $username;
header('location: pshomepage.php');
}
else
{
header('location: invalid.php');
}
exit();
Your problem is your comparison.
if ($_SESSION['username'] != 1)
This is true if $_SESSION['username'] is not set, null, a string, false, etc...
This might be more what you are looking for.
if (isset($_SESSION['username']) && is_string($_SESSION['username']) && strlen($_SESSION['username']))
And you need to fix your SQL injection problem here
$sql = "SELECT * FROM `users` WHERE username='".$username."' and password='".$password."'";
Escape variables with mysql_real_escape_string or use PDO with proper prepared statements.
You should also store passwords as hashes with password_hash(). Fetch the user, compare stored hash to password with password_verify.
if (!password_verify($password, $fetched["password"])) {/* wrong password, show error or something */}