PHP Superglobal $_SESSION

1. What is a PHP Session?

A PHP session is a way to store information on the server for individual users. Unlike cookies, which are stored on the client side, session data is stored on the server, making it more secure. A unique session ID links each user to their stored data.

2. Starting a Session

To use sessions in PHP, you must start the session using the session_start() function before any output is sent to the browser. This initializes or resumes the session.

<?php
// Start the session
session_start();
?>

Always place session_start() at the top of your PHP file.

3. Storing and Accessing Session Data

To store data in a session, use the $_SESSION superglobal array with a key. For example, to store a username:

<?php
session_start();
$_SESSION['username'] = 'JohnDoe';
echo "Username is: " . $_SESSION['username'];
?>

This stores JohnDoe in the session variable $_SESSION['username'], accessible across pages after starting the session.

4. Updating Session Data

Updating session data is as simple as assigning a new value to a session variable:

<?php
$_SESSION['username'] = 'JaneDoe'; // Updates the username
?>

5. Removing Session Data

To remove a specific session variable, use the unset() function. To clear all session data, use session_unset() or assign $_SESSION to an empty array.

<?php
unset($_SESSION['username']); // Remove specific session variable

session_unset(); // Clears all session variables
?>

6. Destroying a Session

To completely end a session and remove all session data, use session_destroy(). This is often done when logging a user out.

<?php
session_destroy(); // Ends the session and clears session data
?>

Note that session_destroy() will not unset the $_SESSION variables until the page is reloaded.

7. Example: Using $_SESSION for User Login

Here's a basic example of using $_SESSION to store a user's login status:

<?php
session_start();

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    // Assume successful login
    $_SESSION['loggedin'] = true;
    $_SESSION['username'] = $_POST['username'];
    echo "Welcome, " . $_SESSION['username'];
}

// Check if user is logged in
if (isset($_SESSION['loggedin']) && $_SESSION['loggedin'] === true) {
    echo "You are logged in as " . $_SESSION['username'];
} else {
    echo "Please log in.";
}
?>

This example checks if the user is logged in by verifying $_SESSION['loggedin'] and displays a message accordingly.

8. Conclusion

The $_SESSION superglobal is a powerful tool for managing user-specific data across pages. By securely storing data on the server, sessions make it easy to build user login systems, shopping carts, and other stateful web applications.