Copy CertCenter.inc.php to your favorite library folder. Here's how you can make a simple function call to the CertCenter REST API.
require_once '../my-lib-folder/CertCenter.inc.php';
use CertCenter\RESTful as ccAPI;
$api = new ccAPI();
$api->setAuthorization('#your-token#');
$limits = $api->Limit();
print_r($limits);
Before you're able to make function calls against the REST api you need to set your credentials. This can either be done by using the constructor:
$api = new ccAPI($AuthorizationToken="#your-token#");
or by calling the public class method:
$api->setAuthorization('#your-token#');
It's quite easy and a howto is already available:
An extensive API and module documentation is available at https://api.certcenter.help