Web Programming Step by Step

Lecture 25
Cookies and Sessions

References: tizag.com sessions, cookies; Codewalkers

Except where otherwise noted, the contents of this presentation are Copyright 2009 Marty Stepp and Jessica Miller.

Stateful client/server interaction

Sites like amazon.com seem to "know who I am." How do they do this? How does a client uniquely identify itself to a server, and how does the server provide specific content to each client?

• HTTP is a stateless protocol; it simply allows a browser to request a single document from a web server
• in these slides, we'll learn about pieces of data called cookies used to work around this problem, which are used as the basis of higher-level sessions between clients and servers

What is a cookie?

• cookie: a small amount of information sent by a server to a browser, and then sent back by the browser on future page requests
• cookies have many uses:
  • authentication
  • user tracking
  • maintaining user preferences, shopping carts, etc.
• a cookie's data consists of a single name/value pair, sent in the header of the client's HTTP GET or POST request

How cookies are sent

• when the browser requests a page, the server may send back a cookie(s) with it
• if your server has previously sent any cookies to the browser, the browser will send them back on subsequent requests

• alternate model: client-side JavaScript code can set/get cookies

Myths about cookies

• Myths:
  • Cookies are like worms/viruses and can erase data from the user's hard disk.
  • Cookies are a form of spyware and can steal your personal information.
  • Cookies generate popups and spam.
  • Cookies are only used for advertising.
• Facts:
  • Cookies are only data, not program code.
  • Cookies cannot erase or read information from the user's computer.
  • Cookies are usually anonymous (do not contain personal information).
  • Cookies CAN be used to track your viewing habits on a particular site.

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.

Where are the cookies on my computer?

• IE: HomeDirectory\Cookies
  • e.g. C:\Documents and Settings\jsmith\Cookies
  • each is stored as a .txt file similar to the site's domain name
• Firefox: HomeDirectory\.mozilla\firefox\???.default\cookies.txt
  • view cookies in Firefox preferences: Privacy, Show Cookies...
    

Cookies in JavaScript

document.cookie = "username=smith;password=12345";

<!-- using the instructor-provided Cookies.js class -->
<script src="Cookies.js" type="text/javascript"></script>

Cookies.set("username", "smith");
...
alert(Cookies.get("username"));   // smith

• JS has a global document.cookie field (a string)
• you can manually set/get cookie data from this field (sep. by ;), and it will be saved in the browser
• we have written a Cookies.js helper class with methods set, get, exists, remove, and remember

Setting a cookie in PHP

setcookie("name", "value");

setcookie("username", "martay");
setcookie("favoritecolor", "blue");

• setcookie causes your script to send a cookie to the user's browser
• setcookie must be called before any output statements (HTML blocks, print, or echo)
• you can set multiple cookies (20-50) per user, each up to 3-4K bytes

Retrieving information from a cookie

$variable = $_COOKIE["name"];   # retrieve value of the cookie

if (isset($_COOKIE["username"])) {
	$username = $_COOKIE["username"];
	print("Welcome back, $username.\n");
} else {
	print("Never heard of you.\n");
}
print("All cookies received:\n");
print_r($_COOKIE);

• any cookies sent by client are stored in $_COOKIES associative array
• use isset function to see whether a given cookie name exists

Setting a persistent cookie in PHP

setcookie("name", "value", timeout);

$expireTime = time() + 60*60*24*7;   # 1 week from now
setcookie("CouponNumber", "389752", $expireTime);
setcookie("CouponValue", "100.00", $expireTime);

• to set a persistent cookie, pass a third parameter for its timeout in seconds
• time function returns the current time in seconds
  • date function can convert a time in seconds to a readable date

Removing a persistent cookie

setcookie("name", "", time() - 1);

setcookie("CouponNumber", "", time() - 1);

• if the server wants to remove a persistent cookie, it should set it again, passing a timeout that is prior to the present time

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
• client sends that same session ID back to server on future requests
• server uses session ID to retrieve the data for the client's session later, like a ticket given at a coat-check room

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

Browsers that don't support cookies

session_start();   # same as usual

# Generate a URL to link to one of our site's pages
# (you probably won't ever need to do this)
$orderUrl = "/order.php?PHPSESSID=" . session_id();

• if a client's browser doesn't support cookies, it can still send a session ID as a query string parameter named PHPSESSID
  • this is done automatically; session_start detects whether the browser supports cookies and chooses the right method
• if necessary (such as to build a URL for a link on the page), the server can find out the client's session ID by calling the session_id 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

Practice problem: remembering query

• Modify the movie.php movie search script from previous lectures so that it remembers the current user's last query (if any), and offers the user a chance to search for it again, such as:
  • Welcome back! Would you like to repeat your recent search for Fight Club?
• Pretend that the movie-search program is running on a system that wants to limit repeated usage by particular users. Add code so that a given user can only conduct one session per day.