What steps can be taken to address the warning message about session side-effects in PHP?

To address the warning message about session side-effects in PHP, you can regenerate the session ID after a user logs in to prevent session fixation attacks. This can be done by calling session_regenerate_id(true) after successful authentication.

// Start or resume session
session_start();

// Authenticate user
if ($authenticated) {
    // Regenerate session ID to prevent session fixation
    session_regenerate_id(true);
    
    // Continue with authenticated user actions
}