What are best practices for including external PHP files in a script to avoid redundancy and potential errors?

When including external PHP files in a script, it is best practice to use the `require_once` or `include_once` functions to avoid redundancy and potential errors caused by including the same file multiple times. This ensures that the file is only included once, preventing any conflicts or redeclaration of functions and classes.

// Include external PHP file using require_once
require_once 'external_file.php';