Including Files in PHP

Let's say we have a file that stores a set of functions:

functions.php<?php
  function square($num) {
    return $num ** 2;
  }
  
  function cube($num) {
    return $num ** 3;
  }
?>

Let's make the functions from this file available in our main file. To do this, include the file with functions using the require operator:

index.php<?php
  require 'functions.php';
?>

After this, in our main file we can use the functions from the included file:

index.php<?php
  require 'functions.php';
  
  echo square(3) + cube(4);
?>