session.php

<?php
// A class to help work with Sessions
// In our case, primarily to manage logging users in and out

// Keep in mind when working with sessions that it is generally 
// inadvisable to store DB-related objects in sessions

class Session {
	
	private $logged_in=false;
	public $user_id;
	public $message;
	public $url;
	
	function __construct() {
		//session_start(); // should be called manual
		$this->check_message();
		$this->check_login();
		if($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 static function start()
        {
            session_start();
        }

        public static function set ($key, $value = null)
        {
            $_SESSION[$key] = $value;
        }

        public static function get ($key)
        {
            if (isset($_SESSION[$key]) {
                return $_SESSION[$key];
            }
            return null;
        } 
	
	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;
	}

	public function message($msg="") {
		if(!empty($msg)) {
			// then this is "set message"
			// make sure you understand why $this->message=$msg wouldn't work
			$_SESSION['message'] = $msg;
		} else {
			// then this is "get message"
			return $this->message;
		}
	}
	public function url($url = "") {
		if(!empty($url)) {
			$_SESSION['url'] = $url;
		} else {
			return $this->url;
		}
	}

	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;
		}
	}
  
	private function check_message() {
		// Is there a message stored in the session?
		if(isset($_SESSION['message'])) {
			// Add it as an attribute and erase the stored version
			$this->message = $_SESSION['message'];
			unset($_SESSION['message']);
		} else {
			$this->message = "";
		}
	}
	
}

$session = new Session();
$message = $session->message();
?>