Why did I get "curl_exec error 6" with the PHP SDK?
You got a cURL error 6 message - Couldn't resolve host.
Version 7.10.x of cURL has a known bug that triggers a curl_exec 6 error.
Check your version of cURL. If necessary, upgrade to a recent version of cURL, such as 7.53.1.
If you use the most recent version of cURL, check your host's ability to find the IP address of a host using DNS.