How can undefined variable errors be avoided when including files with functions in PHP?

When including files with functions in PHP, undefined variable errors can be avoided by using the `include_once` or `require_once` functions instead of `include` or `require`. This ensures that the file is included only once, preventing any potential conflicts with variable declarations.

// Include the file with functions using require_once
require_once 'functions.php';

// Call the function from the included file
myFunction();