What are some potential issues that can arise when including PHP files within each other?

One potential issue that can arise when including PHP files within each other is the risk of variable or function name conflicts. To avoid this, you can use the `require_once` function instead of `require` or `include` to ensure that the file is only included once, preventing any conflicts.

<?php
require_once 'file1.php';
require_once 'file2.php';
// Rest of the code
?>