Single File Inclusion in PHP

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

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

Let's say we use the functions from the file pow.php in the file sum.php:

sum.php<?php
  require 'pow.php';
  
  function squareSum($arr) {
    $res = 0;
    
    foreach ($arr as $elem) {
      $res += square($elem);
    }
    
    return $res;
  }
  
  function cubeSum($arr) {
    $res = 0;
    
    foreach ($arr as $elem) {
      $res += cube($elem);
    }
    
    return $res;
  }
?>

Let's say in the main file we include both files with functions:

index.php<?php
  require 'pow.php';
  require 'sum.php';
  
  echo square(3) + squareSum([1, 2, 3]);
?>

However, we face a problem. The file pow.php will be included twice in the file index.php: once directly and once through the file sum.php.

This will cause a problem because we will have two sets of functions with the same names.

To solve the problem, all files should be included using the require_once operator - it will include the file only once, ignoring repeated inclusions:

index.php<?php
  require_once 'pow.php';
  require_once 'sum.php';
  
  echo square(3) + squareSum([1, 2, 3]);
?>