What is the warning about relying on a session side-effect in PHP 4.2.3?

In PHP 4.2.3, there is a warning about relying on a session side-effect where changes to $_SESSION data may not be immediately visible. To solve this issue, you should call session_write_close() after making changes to the $_SESSION data to ensure that the changes are immediately saved to the session storage. 

<?php
session_start();

// Make changes to $_SESSION data

session_write_close();
?>

Keywords

session side-effect PHP 4.2.3 warning security vulnerability session fixation

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