In PHP, what are some common reasons why session variables may change unexpectedly after a page refresh, particularly when using session_start()?

Session variables may change unexpectedly after a page refresh if the session is not properly initialized using `session_start()` at the beginning of each page that needs to access session data. This can cause the session to be reset or not properly loaded, leading to unexpected changes in session variables. To solve this issue, ensure that `session_start()` is called at the beginning of each page where session variables are used.

<?php
session_start();

// Your PHP code here
?>