How to create custom library in CodeIgniter?

Considering that CI4 will be installed via composer: you can create your libraries without worrying about codeigniter methods and then upload your library to packagist. From within your codeigniter directory, composer require "companyName/yourLibrary" and then voila.

Your library file may start off like

PHP Code:

<?php
namespace MyCompanyName;

class math {
 public function multiply($a = 0, $b = 0) {
   return $a * $b;
 }
} 


then in your controller file, you can do

PHP Code:

public function index() {
 $object = new MyCompanyName\math;
 $answer = $object->multiply(4, 5);

 // code here to pass the variable $answer to the view
}