In PHP, sessions are used to store data that can be accessed across multiple pages during a user’s visit to a website. This allows you to keep track of a user’s actions and preferences, such as login status, shopping cart contents, and so on.

To start a new session, you can use the session_start() function. This function must be called before any output is sent to the browser, typically at the top of your PHP script.

session_start();

Once a session is started, you can store data in the $_SESSION superglobal array. This array works like any other PHP array, you can add, modify, or delete elements using array notation.

$_SESSION['username'] = "John";
$_SESSION['cart'] = array();

To retrieve the value of a session variable, you can simply access it using the $_SESSION superglobal array.

$username = $_SESSION['username'];

To delete a session variable, you can use the unset() function or $_SESSION array.

unset($_SESSION['username']);

$_SESSION['username']=null;

It is also possible to destroy a session completely using the session_destroy() function. This function terminates the current session and releases all session data.

session_destroy();

When working with sessions, it’s important to keep in mind that the data is stored on the server, not on the client’s computer. This means that the data is not accessible to the client and cannot be tampered with, but it also means that you need to take care of session data management and expiration, otherwise it can lead to session hijacking.

It’s also worth mentioning that session data is usually stored in a file on the server, or in a database, by default. You can configure PHP to store session data in a different way using the session_set_save_handler() function.

These are just a few examples of how to use sessions in PHP, it’s recommended to consult the official PHP documentation for more information on working with sessions in PHP.