What best practices should be followed when including multiple PHP files in a project?

When including multiple PHP files in a project, it is important to follow best practices to ensure proper organization and maintainability. One common approach is to use the `require_once` or `include_once` functions to include files only once to prevent conflicts. Additionally, it is recommended to use relative paths for file inclusions to make the project more portable and easier to maintain.

// Example of including multiple PHP files using require_once

require_once 'config.php';
require_once 'functions.php';
require_once 'database.php';