Awesome
php-steam-totp
PHP library for TOTP for use with Steam. It's documented with phpdoc; just read it to learn how to use it.
Only requirement is cURL.
Example
<?php
require_once 'vendor/autoload.php'; // if you're using composer
use SteamTotp\SteamTotp;
echo "Login code: " . SteamTotp::getAuthCode("cnOgv/KdpLoP6Nbh0GMkXkPXALQ=");