Lecture 7 Maintaining State (cookies & sessions) & MySQL Interaction (revisited)

Stateful v. Stateless "State" is a central concern of all sorts of distributed applications, but especially of Web applications. When applied to a protocol, "state" treats each series of interactions as having continuity, much like a single program's state. A "stateless" protocol is one in which there is no such continuity; each request must be processed entirely on its own merits. HTTP and its derivatives are intrinsically "stateless". The request/response cycle of a HTTP interaction does not maintain "memory" of any previous interactions.

Stateful v. Stateless (2) Stateful Interaction: Request 1: “What is Alice’s account number?” Response 1: Request 2: “What is her current balance?” Response 2: £ Stateless Interaction: Request 1: “What is Alice’s account number?” Response 1: Request 2: “What is Alice’s current balance?” Response 2: £345.65

Is PHP stateless? (well … yes) On a webserver, PHP scripts have no shared state, so each instance of a PHP script runs in its own logical memory space. The scripts maintain no persisted state, so each script start off fresh as a daisy, with no data to indicate what happened the previous times it was executed. Variables are destroyed as soon as the page script finishes executing. The script can access the ‘referrer’, the address of the previous page, although this can’t really be trusted. $_SERVER['HTTP_REFERER']

Is PHP stateless? (well … not necessarily) The usual way to maintain state in PHP scripts is via the use of sessions. To understand how these work, we need to have a look at what cookies are and how they work …

Client/Server interaction with Cookies A cookie is a small file that the server embeds on the user's browsers file system. Each time the same browser requests a page, it will send the cookie too. With PHP, you can both create and retrieve cookie values.

Setting / Retrieving / Deleting a Cookie with PHP Setting a cookie : use the setcookie() function setcookie(name, value, expire, path, domain); Retrieve a cookie : use the $_COOKIE superglobal // Print a cookie echo $_COOKIE["name"]; // A way to view all cookies print_r($_COOKIE); Delete a cookie : set the time to a past instance // set the expiration date to one hour ago setcookie("name", "", time()-3600);

Title 1) { echo("This is visit number $visits."); } else { // First visit echo('Welcome to my Website! This is your first visit!'); } ?> Note : the cookie must be sent before any other headers. Setting & Retrieving a Cookie with PHP Run it

setcookie(name [,value [,expire [,path [,domain,secure]]]]]) name = cookie name value = data to store (string) expire = UNIX timestamp when the cookie expires. Default is that cookie expires when browser is closed. path = Path on the server within and below which the cookie is available on. domain = Domain at which the cookie is available for. secure = If cookie should be sent over HTTPS connection only. Default false. setcookie() keys & values

Each cookie on the user’s computer is connected to a particular domain. Each cookie can store up to 4kB of data. A maximum of 20 cookies can be stored on a user’s PC per domain Only strings can be stored in Cookie files. To store an array in a cookie, convert it to a string by using the serialize() PHP function. The array can be reconstructed using the unserialize() function once it had been read back in. Cookies are stored client-side, so never can’t be trusted completely: They can be easily viewed, modified or created by a 3 rd party. They can be turned on and off at will by the user. Cookie limits & notes

PHP Sessions Since HTTP is a stateless protocol – a PHP session can be used to store user information on the server for later use (i.e. username, shopping items, etc). Session information is temporary and will be deleted after the user has left the website. Session data can be made persistent by storing the data in a database. Sessions work by creating a unique id (UID) for each visitor and store variables based on this UID. The UID is either stored in a cookie or is propagated in the URL (if cookies are turned off for instance).

CookiesSessions Limited storage spacePractically unlimited space Insecure storage client-sideReasonably securely stored server-side User controlledNo user control Cookies v. Sessions

session_start(); PHP does all the work: It looks for a valid session id in the $_COOKIE or $_GET superglobals – if found it initializes the data. If none found, a new session id is created. Note that like setcookie(), this function must be called before any echoed output to browser. Example session id: 26fe536a534d3c7cde4297abb45e275a Starting / Resuming a Session

The $_SESSION superglobal array can be used to store any session data. e.g. $_SESSION[‘name’] = $name; $_SESSION[‘age’] = $age; To retrieve session values, data is simply read back from the $_SESSION superglobal array. e.g. $name = $_SESSION[‘name’]; $age = $_SESSION[‘age’]; To delete session data – simply unset() a particular session variable e.g. unset($_SESSION[‘name’]); To destroy a session – use the session_destory() function e.g. session_destory(); Storing / Retrieving / Deleting Session data

Setting & Retrieving a Session value with PHP <?php session_start(); if(isset($_SESSION['visits'])) { $_SESSION['visits']=$_SESSION['visits']+1; } else { $_SESSION['visits']=1; } echo "This is visit number ". $_SESSION['visits']; ?> Run it

Typical process flow to save session data in a DB

PHP & DB Interaction with MySQL Open Source (relational) database server Runs on many platforms (Unix & Windows) Networked server – no fancy GUI like MS Access. You can find clients (such as phpMyAdmin) that provide a GUI. cems : (note: only available within uwe) Great for small, medium to large-sized applications (ebay, amazon, facebook etc. all make use of it) MySQL

phpMyAdmin A MySQL client written in PHP Via a browser you can manage: Manage Databases Manage MySQL users Submit queries (SQL) A great way to learn SQL!

There are three main API options when considering connecting to a MySQL database server: PHP's MySQL Extension - original extension which provides a procedural interface and is intended for use only with MySQL versions older than Can be used with versions of MySQL or newer, but not all of the latest MySQL server features will be available. PHP's mysqli Extension - MySQL improved extension takes advantage of new features found in MySQL versions and newer. The mysqli extension is included with PHP versions 5 and later. PHP Data Objects ( PDO ) - PHP Data Objects, or PDO, is a database abstraction layer that provides a consistent API regardless of the type of database server. In theory, it allows for the switch of the database server, from say Firebird to MySQL, with only minor changes the PHP code. PHP (main) API’s for using MySQL

Advantages of the mysqli API Object-oriented interface Support for Prepared Statements Support for Multiple Statements Support for Transactions Enhanced debugging capabilities Embedded server support Note: If using MySQL versions or later it is strongly recommended that the mysqli extension is used.

qidquoteauthordobdodurlcategory 1There is no remedy but to love more. Henry David Thoreau http://en.wikipedia.org/w iki/Henry_David_Thorea u love 6Work is the curse of the drinking classes. Oscar Wilde http://en.wikipedia.org/w iki/Oscar_wilde humour 11Religion is what keeps the poor from murdering the rich. Napoleon Bonaparte http://en.wikipedia.org/w iki/Napoleon politics example MySQL db Entity Model Example records (3)

// Connect to the db $mysqli = new mysqli('hostname','username','password','database'); //Send the query to the database and pull the records in a // certain category using the SELECT statement // If the result returns true if ($result = $mysqli->query("SELECT quote, url FROM quote WHERE category='love'")) { // print out the number of records retrieved echo 'For the category "love", there are '.$result->num_rows.' records. '; // The "fetch_object()" method allows access to the returned // rows within the resource object ($result in this case). while ($row = $result->fetch_object()) { echo 'Quote: '.$row->quote.' '; echo 'URL: url.'>'.$row->url. ' '; } else { // it’s an error & the query failed echo $mysqli->error; } // end else $mysqli->close(); example script using mysqli Run it

Update / delete/ add new record using mysqli – left as an exercise