What potential pitfalls can arise when using session variables in PHP for user authentication and access control?

One potential pitfall when using session variables for user authentication is the risk of session hijacking or fixation if the session ID is not properly managed. To mitigate this risk, it's important to regenerate the session ID after a successful login or at regular intervals. This helps prevent unauthorized access to the user's session.

// Start the session
session_start();

// Regenerate the session ID to prevent session fixation
session_regenerate_id(true);