Run Magento 2 Code Externally

Working with Magento 2 its pretty usual, that you often needs to test or run any piece of code outside Magento. To do so, the following snippet will help you:

<?php

use Magento\Framework\App\Bootstrap;

require __DIR__ . '/app/bootstrap.php';

$bootstrap = Bootstrap::create(BP, $_SERVER);

$objectManager = $bootstrap->getObjectManager();

$state = $objectManager->get('Magento\Framework\App\State')->setAreaCode('frontend');

// Your Magento 2 code here

Additional notes:

The area code is frontend, but you change ‘frontend’ to ‘adminhtml’ to run code as if you’re in the Admin.
You have the Object Manager in $objectManager so you can retrieve any other objects that you require.
If the file is going to be placed in a different folder to the Magento 2 root folder, simply modify the path to app/bootstrap.php.

Enjoy!

Source: https://fishpig.co.uk/magento/tutorials/run-magento-code-externally/