Single File Inclusion in PHP

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

<?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:

<?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:

<?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:

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

Create several files with useful sets of functions. Connect these files to each other and to your main file.