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.

See also:
cURL website