<?php

    session_start();

    function hash($value)
    {
        $salt = "S?@!%)";
        if (is_array($value))
        {
            return md5($salt.serialize($value));
        } else
        {
            return md5($salt.$value);
        }
    }

    if (isset($_POST['username']))
    {
        $username   = mysql_real_escape_string($_POST['username']);
        $password   = hash($_POST['password']);

        $sql        = "SELECT * FROM users WHERE username='".$username."' AND password='".$password."'";
        $query      = mysql_query($sql);
        if (mysql_num_rows($query) > 0)
        {
            $user   = mysql_fetch_array($query);
            $_SESSION['hash']   = hash($user);
            $_SESSION['user]    = $user;
        } else
        {
            echo "Incorrect Login.";
        }
    } else if (isset($_GET['logout']))
    {
        session_destroy();
        header("Location: index.php");
    }

    if (isset($_SESSION['user']) AND $_SESSION['hash'] == hash($_SESSION['user']))
    {
        echo "Welcome ".$_SESSION['user']['username'];
    } else
    {
        echo "Welcome Guest!";
    }

?>