Web Programming Step by Step, 2nd Edition

Lecture 13: Sessions

Reading: 14.3 - 14.4

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

How long does a cookie exist?

• session cookie : the default type; a temporary cookie that is stored only in the browser's memory
  • when the browser is closed, temporary cookies will be erased
  • can not be used for tracking long-term information
  • safer, because no programs other than the browser can access them
• persistent cookie : one that is stored in a file on the browser's computer
  • can track long-term information
  • potentially less secure, because users (or programs they run) can open cookie files, see/change the cookie values, etc.

What is a session?

• session: an abstract concept to represent a series of HTTP requests and responses between a specific Web browser and server
  • HTTP doesn't support the notion of a session, but PHP does
• sessions vs. cookies:
  • a cookie is data stored on the client
  • a session's data is stored on the server (only 1 session per client)
• sessions are often built on top of cookies:
  • the only data the client stores is a cookie holding a unique session ID
  • on each page request, the client sends its session ID cookie, and the server uses this to find and retrieve the client's session data

How sessions are established

• client's browser makes an initial request to the server
• server notes client's IP address/browser, stores some local session data, and sends a session ID back to client (as a cookie)
• client sends that same session ID (cookie) back to server on future requests
• server uses session ID cookie to retrieve its data for the client's session later (like a ticket given at a coat-check room)

Cookies vs. sessions

• duration: sessions live on until the user logs out or closes the browser; cookies can live that long, or until a given fixed timeout (persistent)
• data storage location: sessions store data on the server (other than a session ID cookie); cookies store data on the user's browser
• security: sessions are hard for malicious users to tamper with or remove; cookies are easy
• privacy: sessions protect private information from being seen by other users of your computer; cookies do not

Sessions in PHP: session_start

session_start();

• session_start signifies your script wants a session with the user
  • must be called at the top of your script, before any HTML output is produced
• when you call session_start:
  • if the server hasn't seen this user before, a new session is created
  • otherwise, existing session data is loaded into $_SESSION associative array
  • you can store data in $_SESSION and retrieve it on future pages
• complete list of PHP session functions

Accessing session data

$_SESSION["name"] = value;        # store session data
$variable = $_SESSION["name"];     # read session data
if (isset($_SESSION["name"])) {  # check for session data

if (isset($_SESSION["points"])) {
	$points = $_SESSION["points"];
	print("You've earned $points points.\n");
} else {
	$_SESSION["points"] = 0;  # default
}

• the $_SESSION associative array reads/stores all session data
• use isset function to see whether a given value is in the session

Where is session data stored?

• on the client, the session ID is stored as a cookie with the name PHPSESSID
• on the server, session data are stored as temporary files such as /tmp/sess_fcc17f071...
• you can find out (or change) the folder where session data is saved using the session_save_path function
• for very large applications, session data can be stored into a SQL database (or other destination) instead using the session_set_save_handler function

Session timeout

• because HTTP is stateless, it is hard for the server to know when a user has finished a session
• ideally, user explicitly logs out, but many users don't
• client deletes session cookies when browser closes
• server automatically cleans up old sessions after a period of time
  • old session data consumes resources and may present a security risk
  • adjustable in PHP server settings or with session_cache_expire function
  • you can explicitly delete a session by calling session_destroy

Ending a session

session_destroy();

• session_destroy ends your current session
• potential problem: if you call session_start again later, it sometimes reuses the same session ID/data you used before
• if you may want to start a completely new empty session later, it is best to flush out the old one:

  session_destroy();
  session_regenerate_id(TRUE);   # flushes out session ID number
  session_start();
  

Common session bugs

• session_start doesn't just begin a session; it also reloads any existing session for this user. So it must be called in every page that uses your session data:

  # the user has a session from a previous page
  print $_SESSION["name"];   # undefined
  
  session_start();
  print $_SESSION["name"];   # joe
  

• previous sessions will linger unless you destroy them and regenerate the user's session ID:

  session_destroy();
  session_regenerate_id(TRUE);
  session_start();
  

Practice problem: Power Animal

• Write a page poweranimal.php that chooses a random "power animal" for the user.
• The page should remember what animal was chosen for the user and show it again each time they visit the page.
• It should also count the number of times that user has visited the page.
• If the user selects to "start over," the animal and number of page visits should be forgotten.

Implementing user logins

• many sites have the ability to create accounts and log in users
• most apps have a database of user accounts
• when you try to log in, your name/pw are compared to those in the database

"Remember Me" feature

• How might an app implement a "Remember Me" feature, where the user's login info is remembered and reused when the user comes back later?
• Is this stored as session data? Why or why not?
• What concerns come up when trying to remember data about the user who has logged in?