This error indicates that curl is failing due to an HTTPS certificate validation error. (curl is used by the PHP SDK when PHP <5.3.7 to fetch HTTP(S) URLs.)
However the PHP SDK explicitly disables certificate validation in Http.php:159, so this error is rather unexpected:
private function requestWithCurl(
...
CURLOPT_SSL_VERIFYPEER => FALSE,
According to the PHP docs (http://php.net/manual/en/function.curl-setopt.php), CURLOPT_SSL_VERIFYPEER, is not limited to any particular PHP version, so it is presumably available in PHP 5.3.3. Therefore it is probably getting passed through correctly to curl.
Perhaps your curl is configured in some kind of strict mode that always performs certificate validation even when instructed not to?
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