Just so that this question doesn't continue to show up as unanswered in an auto-email, I'm going to add your answer from your update here -- hope that's alight!
file_get_contents
doesn't seem to be a good method to access the oauth page, I used curl instead:
$request_url = 'https://www.eventbrite.com/oauth/token';
$ch = curl_init();
curl_setopt($ch, CURLOPT_POST, TRUE);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($params));
curl_setopt($ch, CURLOPT_URL, $request_url);
curl_setopt($ch, CURLOPT_HEADER, FALSE);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
$response = curl_exec($ch);
curl_close($ch);
Thanks for making it easy by answering your own question! ;)