Lesson 4.10. PHP sessions

Introduction

PHP variables declared inside a script only exist during execution of the script (i.e. for a given page). At the end of the execution, the content of the variables is definitively lost. It is often necessary to have persistent data during navigation. On a merchant site for example, the basket is kept even if the user navigates to a new page or reloads the current page. The PHP sessions allow you to keep this data during navigation from one page to another.

Initialization

A PHP session alawys starts with the call of the session_start () function. The call of this function must imperatively be the very first instruction of your script or document, otherwise the sessions will not work. Here is an example :

<?php
// Starts sessions
session_start();
?>
// Your HTML or PHP code

Session variables

Data stored in session variables can be accessed via the global table $ _SESSION. This array can be read and write. The following example counts the number of times the page has been loaded:

<?php
// Start sessions
session_start();

// If the session already exists, increments the counter
// otherwise initialize the counter
if (isset($_SESSION['counter'])) 
  $_SESSION['counter']++;
else
  $_SESSION['counter']=1;
?>

<!-- Display counter -->
You visited this page <?= $_SESSION['counter'] ?> times.

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Destroy

Sessions, as their name suggests, are linked to a user session. When the user closes his browser, the session is automatically destroyed. If the sessions are inactive, they are automatically destroyed after a given time lapse. This lifetime is a parameter of the server, generally set by default on 24 minutes.

It is often necessary to destroy all or part of the session variables. Typicaly when a user logs out. There are two ways to destroy session variables.

Destroy a given variable

The unset function allows you to destroy a variable while keep the others:

unset( $_SESSION['counter'] );

Destroy all variables

The session_unset destroy all the session variables:

session_unset();

This function is equivalent to $_SESSION = array();.

Exercice

Complete the following code so that it displays the number of visits for pages page1.php and page2.php:

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Solution

Lesson 4.9 Summary Lesson 5.1