What are common pitfalls when including multiple PHP files in an index.php file?

Common pitfalls when including multiple PHP files in an index.php file include namespace collisions, duplicate function or class declarations, and unexpected variable reassignments. To avoid these issues, it's important to properly structure your included files and use include_once or require_once to prevent multiple inclusions.

<?php
include_once 'file1.php';
include_once 'file2.php';
include_once 'file3.php';
// rest of the code
?>