PHP logging out

I am doing the Lynda.com learning PHP 2 videos and have run into a problem, in that the instructor seems to have neglected to tell us one of the steps he does in the video. I have uploaded the relevant video here http://www.youtube.com/watch?v=fFKgAa7RAjo but will also describe the problem. At 6:40 of the video, after logging in to our application, he arrives at public/admin/index.php which has two links on it. one link allows him to "view log file" which takes him to public/admin/logfile.php and the other link allows him to log out. He doesn't tell us how to make these links. I can obviously make a link to view logfile

 <a href="logfile.php">View Logfile</a>

but I don't know how to make the link that will log me out, because that will obviously involve some PHP.

I have included below the login.php file, the index.php file (it's redirected to index.php after logging in) and the functions.php file. Do you know how I would logout from this?

This is the login.php file

<?php

require_once("../../includes/initialize.php");

if($session->is_logged_in()){
    redirect_to("index.php");
}

//Remember to give your form's submit tag a name="submit" attribute
if (isset($_POST['submit'])) {//Form has been submitted.

$username = trim($_POST['username']);
$password = trim($_POST['password']);

//Check database to see if username/password exist

$found_user = User::authenticate($username, $password);

if ($found_user) {
    $session->login($found_user);
    log_action('Login', "{$found_user->username} logged in.");
    redirect_to("index.php");
} else {
    //username/password combo was not found in the database
    $message = "Username/password combination incorrect.";
} 
} else {//Form has not been submitted.
    $username = "";
    $password = "";
    }
?>

<?php include_layout_template('admin_header.php'); ?>

        <h2>Staff Login</h2>
        <?php echo output_message($message); ?>

        <form action="login.php" method="post">
            <table>
                <tr>
                    <td>Username:</td>
                    <td>
                        <input type="text" name="username" maxlength="30" value="<?php
                        echo htmlentities($username); ?>" />
                    </td>
                </tr>
                <tr>
                    <td>Password:</td>
                    <td>
                        <input type="password" name="password" maxlength="30" value="<?php
                        echo htmlentities($password); ?>" />
                    </td>
                </tr>
                <tr>
                    <td colspan="2">
                        <input type="submit" name="submit" value="login" />
                    </td>
                </tr>
            </table>
        </form>
        </div>
        <?php include_layout_template('admin_footer.php'); ?>

Functions.php

<?php

function strip_zeros_from_date( $marked_string=""){
//first remove the marked zeros
$no_zeros = str_replace('*0', '', $marked_string);
//then remove any remaining marks
$cleaned_string = str_replace('*', '', $no_zeros);
return $cleaned_string;

}

function redirect_to( $location= NULL) {
    if($location != NULL) {
    header("Location: {$location}");
    exit;
    }

}

function output_message($message=""){
if (!empty($message)) {
return "<p class=\"message\">{$message}</p>";
} else {
    return "";
    }
}

function __autoload($class_name) {
  $class_name = strtolower($class_name);
  $path = LIB_PATH.DS."{$class_name}.php";
  if(file_exists($path)){
  require_once($path);
  } else {
  die("The file {$class_name}.php could not be found.");
  }
}

function include_layout_template($template=""){
include(SITE_ROOT.DS.'public'.DS.'layouts'.DS.$template);
}

function log_action($action, $message=""){
$logfile = SITE_ROOT.DS.'logs'.DS.'log.txt';
$new = file_exists($logfile) ? false : true;
if($handle = fopen($logfile, 'a')) { //apppend
$timestamp = strftime("%Y-%m-%d %H:%M:%S", time());
$content = "{$timestamp} | {$action}: {$message}\n";
fwrite($handle,$content);
fclose($handle);
if($new) {chmod($logfile, 0755); }
} else {
 echo "Could not open log file for writing.";
}
}

?>

Index.php

<?php

require_once('../../includes/initialize.php');

if (!$session->is_logged_in()) { redirect_to("login.php"); }
?>

<?php include_layout_template('admin_header.php'); ?>

        <h2>Menu</h2>

        </div>


<?php include_layout_template('admin_footer.php'); ?>

Update

Initialize.php

<?php

//Directory_separator is a PHP pre-defined constant
// (\ for windows, / for Unix)

defined('DS') ? null : define('DS', DIRECTORY_SEPARATOR);

defined('SITE_ROOT') ? null : 
 define('SITE_ROOT', DS.'hsphere'.DS.'local'.DS.'home'.DS.'c263430'.DS.'quoralist.com');
// define('SITE_ROOT', realpath(dirname(__FILE__).'/../'));

 //echo SITE_ROOT."<br/>";

 defined('LIB_PATH') ? null : define('LIB_PATH', SITE_ROOT.DS.'includes');
// die(LIB_PATH);

 //echo LIB_PATH."<br/>";

require_once(LIB_PATH.DS."config.php");
require_once(LIB_PATH.DS."functions.php");
require_once(LIB_PATH.DS."session.php");
require_once(LIB_PATH.DS."database.php");
require_once(LIB_PATH.DS."database_object.php");
require_once(LIB_PATH.DS."user.php");

//echo("You die here");

?>

User.php

<?php

require_once(LIB_PATH.DS.'database.php');

class User extends DatabaseObject{

protected static $table_name="users";
public $id;
public $username;
public $password;
public $first_name;
public $last_name;

public function full_name() {
if(isset($this->first_name) && isset($this->last_name)) {
return $this->fi开发者_运维技巧rst_name . " " . $this->last_name;
} else {
  return "";
}
}

public static function authenticate($username="",$password="") {
global $database;
$username = $database->escape_value($username);
$password = $database->escape_value($password);
$sql = "SELECT * FROM users ";
$sql .= "WHERE username = '{$username}' ";
$sql .= "AND password = '{$password}' ";
$sql .= "LIMIT 1";
$result_array = self::find_by_sql($sql);
return !empty($result_array) ? array_shift($result_array) : false; 

}

//common database methods

public static function find_all(){
return self::find_by_sql("SELECT * FROM ".self::$table_name);

}

public static function find_by_id($id=0) {
global $database;
$result_array = self::find_by_sql("SELECT * FROM ".self::$table_name." WHERE id={$id} LIMIT 1");
return !empty($result_array) ? array_shift($result_array) : false; 
}

public static function find_by_sql($sql=""){
global $database;
$result_set = $database->query($sql);
$object_array = array();
while ($row = $database->fetch_array($result_set)) {
$object_array[] = self::instantiate($row);
}
return $object_array;
}




private static function instantiate($record){

$object = new self;
//$object->id = $record['id'];
//$object->username = $record['username'];
//$object->password = $record['password'];
//$object->first_name = $record['first_name'];
//$object->last_name = $record['last_name'];

foreach($record as $attribute=>$value) {
if($object->has_attribute($attribute)) {
$object->$attribute = $value;
}
}
return $object;
}

private function has_attribute($attribute) {
$object_vars = get_object_vars($this);
return array_key_exists($attribute, $object_vars);
}

}




?>

Session.php

<?php


class Session {

    private $logged_in=false;
    public $user_id;

    function __construct() {
    session_start();
    $this->check_login();
    if($this->logged_in){
    //actions to take right away if user is logged in
    } else {
    //actions to take right away if user is not logged in
    }
    }

    public function is_logged_in() {
    return $this->logged_in;
    }

    public function login($user) {
    //database should find user based on username/password
    if($user){
    $this->user_id = $_SESSION['user_id'] = $user->id;
    $this->logged_in = true;
    }
    }

    public function logout(){
    unset($_SESSION['user_id']);
    unset($this->user_id);
    $this->logged_in = false;
    }

    private function check_login(){
    if(isset($_SESSION['user_id'])){
    $this->user_id = $_SESSION['user_id'];
    $this->logged_in = true;
    } else {
     unset($this->user_id);
     $this->logged_in = false;
    }
    }
}

$session = new Session();

?>

 <?php
    session_start();
    session_destroy();
 ?>

That should destroy all variables stored in the session. It is really primitive logging out, but it should work. After you do that just redirect to "index.php" or whatever page you want.