gistfile1.php

<?php
// ...
public function cancel() {
    $endpoint = self::$endpoint . '/' . $this->id . '/cancel';
    return new self($this->client, $this->client->request('put', $endpoint));
}

public function refund() {
    $endpoint = self::$endpoint . '/' . $this->id . '/refund';
    return new self($this->client, $this->client->request('post', $endpoint));
}
//...
?>