Library used in Rockujemy themes and plugins.
Open console in your WordPress theme folder or plugin folder:
cd wordpress\wp-content\themes\your-theme
or
cd wordpress\wp-content\plugins\your-plugin
Run Composer to install this package in your project:
composer require grzegorz-jamroz/wp-extensions
Remember to require vendor/autoload.php
file in your code to enable the class autoloading mechanism provided by Composer.
You can do it for example in the beginning of:
wordpress/wp-content/themes/your-theme/functions.php
require_once __DIR__ . '/vendor/autoload.php';