Use OAuth to download an image attachment.

image download.php

<?php
$oauth = new OAuth($consumer_key, $consumer_secret);
$oauth->setToken($access_token, $access_token_secret);

$oauth->disableRedirects();
$oauth->fetch('https://planningcenteronline.com/attachments/23128559');

$response_info = $oauth->getLastResponseInfo();
copy($response_info['redirect_url'], './file.jpg');
?>