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Sessions
Sessions are a feature in PHP that allow you to store data server side about a user. When a session is setup, a browser cookie is set which identifies the user to PHP so the PHP knows which server side variables to access.
Starting A Session
On every page you want to access the session you will need to start (or load) the session. To do so run the session_start() function which loads the PHP Session System.
<?php
session_start();
Please note, that when using cookie-based sessions, session_start() must be called before outputing anything to the browser. anything else will result in an error.
Accessing And Setting Data In A Session
The $_SESSION['key'] variable is a special type of array (using a browser cookie to determine which session to access).
In the below example you see the user's choice of theme is set to theme number one.
<?php
session_start();
$_SESSION['themechoice'] = 1;
Accessing a session variable is similar to setting one. Simply include the variable where it needs to be accessed. For example echoing it out as shown in the code example below.
<?php
session_start();
echo $_SESSION['themechoice'];
Removing A Session
To remove a session from the system run the following PHP code. It will unset the session variables and delete it from the system.
<?php
session_unset();
session_destroy();
Here's a full example to manually expire a user's session:
<?php
//Start our session.
session_start();
//Expire the session if user is inactive for 30
//minutes or more.
$expireAfter = 30;
//Check to see if our "last action" session
//variable has been set.
if(isset($_SESSION['last_action'])){
//Figure out how many seconds have passed
//since the user was last active.
$secondsInactive = time() - $_SESSION['last_action'];
//Convert our minutes into seconds.
$expireAfterSeconds = $expireAfter * 60;
//Check to see if they have been inactive for too long.
if($secondsInactive >= $expireAfterSeconds){
//User has been inactive for too long.
//Kill their session.
session_unset();
session_destroy();
}
}
//Assign the current timestamp as the user's
//latest activity
$_SESSION['last_action'] = time();
Sessions Are Temporary
It is important to not treat a session as permanent storage. They get cleared from time to time by the developer, whenever the application is moved to a new host server, by the application itself (for example a logout button), and even during server maintenance. For long term storage of data make sure to use a database.
Security
Last but not least it's important to use php sessions securely. Read our article on Session Identifier Acquirement and Session Hijacking for more information.