What are the best practices for including files in PHP to prevent unwanted output?

When including files in PHP, it's important to prevent unwanted output by using the `include_once` or `require_once` functions instead of `include` or `require`. This ensures that the file is only included once, preventing any duplicate output. Additionally, you can encapsulate the included file in a function or class to further isolate its output from the rest of the script.

// Use require_once to include the file
require_once 'file.php';