How can a beginner in PHP programming effectively manage and include external functions in their scripts for optimal performance and functionality?

To effectively manage and include external functions in PHP scripts, beginners can create separate files for their functions and then include them in their main script using the `require` or `include` statement. This helps maintain a clean and organized code structure, improve reusability, and enhance overall performance.

// external_functions.php
function myFunction() {
    // Function logic here
}

// main_script.php
require 'external_functions.php';

// Call the function
myFunction();