What are common mistakes to avoid when creating a script to display online users in PHP?

One common mistake to avoid when creating a script to display online users in PHP is not properly handling sessions. It's important to correctly set session variables when a user logs in and update them when they log out. Additionally, make sure to properly sanitize and validate user input to prevent any security vulnerabilities.

// Start the session
session_start();

// Set a session variable when a user logs in
$_SESSION['user_id'] = $user_id;

// Update the session variable when a user logs out
unset($_SESSION['user_id']);

// Sanitize and validate user input
$username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
$password = filter_var($_POST['password'], FILTER_SANITIZE_STRING);