Facebook Auth with high traffic sites: empty access tokens, empty /me - facebook

Question

We have an app running on a Facebook tab at the moment which is receiving a good deal of traffic. People are signing up every few seconds, and most are successful. However I am running into the following problems:
- access token not received at all (empty response, no error)
- or if it is received, then API call to /me fails (empty response, no error)
EDIT: Apparently limiting of API calls is not the issue, since the 600/600 calls/sec is per user which makes a lot more sense :).
Is anyone else experiencing such issues? I am getting about 20-30 successful app signups a minute and about 2-3 failing ones. Note: these are not users who deny access to the app - those are handled elsewhere...
EDIT: I am getting "failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request"
$token_url = "https://graph.facebook.com/oauth/access_token?client_id=".$oauth_clientid."&redirect_uri=".urlencode($redirect_url)."&client_secret=".$oauth_secret."&code=".$code;
$access_token = file_get_contents($token_url);

Answer 1

Update: After modifying my code to use curl, there was some improvement, but it did not resolve the issue. The application in question is now past its peak usage, so obviously the number of errors have gone down drastically. It still happens sometimes. The errors are (most often):
Curl error (6): Couldn't resolve host 'graph.facebook.com'
Curl error (28): SSL connection timeout
...and other similar errors.
From my experience with a week or two of data, it seems to effect about a percent of requests when there is high traffic, less when there is low - but this is quite unscientific. :)
--
So the solution? Well there is none really. It seems that some percent of requests will fail and my (your) application needs to handle these errors as elegantly as possible.
My only question is, has anyone else had such experience with high traffic apps? (meaning in this case 100 000-500 000 daily pageviews and 50 000-100 000 active users)

Answer 2

Try to log the curl error and return value when you obtain the access_token the answer should be there.

Answer 3

I think it's because php_openssl extension is either absent or disabled. See this question to enable it. Or you can use cURL. Take fb_ca_chain_bundle.crt from PHP SDK official (or set CURLOPT_SSL_VERIFYPEER to false):
$base_url = 'https://graph.facebook.com/oauth/access_token';
$params = array();
$params['client_id'] = $client_id;
$params['client_secret'] = $client_secret;
$params['redirect_uri'] = $redirect_uri;
$params['code'] = $code;
$options = array(
CURLOPT_SSL_VERIFYHOST => 2,
CURLOPT_SSL_VERIFYPEER => true,
CURLOPT_CAINFO => __DIR__ .'/fb_ca_chain_bundle.crt',
CURLOPT_RETURNTRANSFER => true,
CURLINFO_HEADER_OUT => false,
CURLOPT_HEADER => false,
CURLOPT_URL => $base_url . '?' . http_build_query($params),
);
// Init cURL and send the request
$ch = curl_init();
curl_setopt_array($ch, $params);
$result = curl_exec($ch);
$infos = curl_getinfo($ch);
curl_close($ch);
var_dump($result, $infos);
EDIT: i warn you this is a quick copy and paste from my code so you would double check $options.