PHP Sessions
PHP Sessions
A session is a way to store information (in variables) to be used across multiple pages. Unlike a cookie, the information is not stored on the users computer.
When you work with an application, you open it, do some changes, and then you close it. This is much like a Session. The computer knows who you are. It knows when you start the application and when you end. But on the internet there is one problem: the web server does not know who you are or what you do, because the HTTP address doesn't maintain state.
So; Session variables hold information about one single user, and are available to all pages in one application. If you need a permanent storage, you may want to store the data in a database.
An alternative way to make data accessible across the various pages of an entire website is to use a PHP Session. A session creates a file in a temporary directory on the server where registered session variables and their values are stored. This data will be available to all pages on the site during that visit.
The location of the temporary file is determined by a setting in the php.ini file called session.save_path. Before using any session variable make sure you have setup this path.
When a session is started following things happen −
- PHP first creates a unique identifier for that particular session which is a random string of 32 hexadecimal numbers such as 3c7foj34c3jj973hjkop2fc937e3443.
- A cookie called PHPSESSID is automatically sent to the user's computer to store unique session identification string.
- A file is automatically created on the server in the designated temporary directory and bears the name of the unique identifier prefixed by sess_ ie sess_3c7foj34c3jj973hjkop2fc937e3443.
When a PHP script wants to retrieve the value from a session variable, PHP automatically gets the unique session identifier string from the PHPSESSID cookie and then looks in its temporary directory for the file bearing that name and a validation can be done by comparing both values.
A session ends when the user loses the browser or after leaving the site, the server will terminate the session after a predetermined period of time, commonly 30 minutes duration.
Start a PHP Session
A session is started with the session_start() function. Session variables are set with the PHP global variable: $_SESSION. Now, let's create a new page called "demo_session1.php". In this page, we start a new PHP session and set some session variables:
The session_start() function must be the very first thing in your document. Before any HTML tags.
`
<?php
// Start the session
session_start();
// Set session variables
$_SESSION["favcolor"] = "green";
$_SESSION["favanimal"] = "cat";
echo "Session variables are set.";
?>
Get PHP Session Variable Values
Next, we create another page called "demo_session2.php". From this page, we will access the session information we set on the first page ("demo_session1.php").
Notice that session variables are not passed individually to each new page, instead they are retrieved from the session we open at the beginning of each page (session_start()).
Also notice that all session variable values are stored in the global $_SESSION variable:
<?php
session_start();
// Echo session variables that were set on previous page
echo "Favorite color is " . $_SESSION["favcolor"] . ".
";
echo "Favorite animal is " . $_SESSION["favanimal"] . ".";
?>
Another way to show all the session variable values for a user session is to run the following code:
<?php
session_start();
print_r($_SESSION);
?>
How does it work? How does it know it's me? Most sessions set a user-key on the user's computer that looks something like this: 765487cf34ert8dede5a562e4f3a7e12. Then, when a session is opened on another page, it scans the computer for a user-key. If there is a match, it accesses that session, if not, it starts a new session.
`Modify a PHP Session Variable
To change a session variable, just overwrite it:
`
<?php
session_start();
// to change a session variable, just overwrite it
$_SESSION["favcolor"] = "yellow";
print_r($_SESSION);
?>
Destroy a PHP Session
To remove all global session variables and destroy the session, use session_unset() and session_destroy():
<?php
session_start();
// remove all session variables
session_unset();
// destroy the session
session_destroy();
?>
