What are potential pitfalls when using SESSION variables to store form data in PHP?

Potential pitfalls when using SESSION variables to store form data in PHP include security risks such as session hijacking or session fixation. To mitigate these risks, it is important to properly sanitize and validate user input before storing it in SESSION variables. Additionally, it is recommended to regenerate the session ID after a user logs in or performs any sensitive actions.

// Start the session
session_start();

// Sanitize and validate form data before storing in SESSION
$_SESSION['username'] = filter_var($_POST['username'], FILTER_SANITIZE_STRING);

// Regenerate the session ID
session_regenerate_id(true);