In what ways can the inclusion of external PHP files, like in the case of the popup.php script, impact the setting and functionality of cookies in a PHP-based website?

When including external PHP files, like the popup.php script, it can impact the setting and functionality of cookies in a PHP-based website if the external file also tries to set cookies. This can lead to conflicts or unexpected behavior with cookies. To solve this issue, you can use the PHP `ob_start()` and `ob_end_flush()` functions to buffer the output of the included file and prevent any headers (including cookie settings) from being sent prematurely.

<?php
ob_start();
include 'popup.php';
ob_end_flush();
?>