What are the advantages and disadvantages of using sessions versus JavaScript for tracking user activity on a website?

When tracking user activity on a website, using sessions in PHP is a server-side solution that allows for storing and accessing user data across multiple pages. On the other hand, using JavaScript for tracking user activity is a client-side solution that can provide real-time tracking and analytics. PHP Code Snippet:

// Start a session
session_start();

// Store user activity data in session variables
$_SESSION['user_id'] = 123;
$_SESSION['last_activity'] = time();

// Retrieve user activity data from session
$user_id = $_SESSION['user_id'];
$last_activity = $_SESSION['last_activity'];