It turns out that "since=30 days ago" was giving a 400 (bad request) response. This error was not showing up in the CURL errors. Only via
curl_getinfo($this -> ch,CURLINFO_HTTP_CODE);
When i changed
$period = "{$period} days ago";
to
$period = urlencode("{$period} days ago");
//or
$period = strtotime("- {$period} days");
everything worked via CURL.