What are the potential security risks associated with relying on session side-effects in PHP scripts?

Relying on session side-effects in PHP scripts can pose security risks such as session fixation attacks or session hijacking. To mitigate these risks, it is recommended to regenerate the session ID after a successful login or whenever the user's privilege level changes.

// Regenerate session ID after successful login
session_start();
if ($login_successful) {
    session_regenerate_id(true);
}