How does the statelessness of the HTTP protocol affect data storage and retrieval in PHP sessions?

The statelessness of the HTTP protocol means that each request is independent and does not retain information from previous requests. This can make it challenging to store and retrieve data in PHP sessions because the server needs a way to identify and associate each user's session data. One way to solve this issue is by using session cookies, which can store a unique identifier on the client side that is used to retrieve the corresponding session data on the server side.

// Start a session
session_start();

// Set a session cookie with a unique identifier
$session_id = session_id();
setcookie('session_id', $session_id, time() + 3600, '/');

// Store data in the session
$_SESSION['username'] = 'JohnDoe';

// Retrieve data from the session
$username = $_SESSION['username'];