The oauthclient makeOAuthCall() should return response headers, e.g. to get the content type or any other header that the client may need. The easiest I think is to add an extra &$responseHeaders parameter defaulted to null - if set, use the curl_setopt($ch, CURLOPT_HEADERFUNCTION, ...) to get all response headers and add them to the array. Thanks!

See a good implementation https://stackoverflow.com/a/41135574/177275 (2nd answer) - that seem to properly parse response headers.