to get up and running:
the oauth2client uses httpconnection and httpresponse which wrapps the curl php bindings.
if you are using a server with apt you can install it by
apt-get install php5-curl
$client = new OAuth2Client(array('base_url' => 'https://api.flattr.com/rest/v2')); $thing = $client->getParsed('/things/423405/'); var_dump($thing); // array
create an API key at https://flattr.com/apps
make sure you add a correct callback_url it should be something like http://url-to-your-vhost/callback.php
Copy /config.template.php to /config.php and enter your api credentials.
Point a apache/lighttpd/nginx vhost to the minimal/ directory and restart.
sample app that implements a connect with flattr (based on sessions, no database, using coltrane framework)