mailgun-php error setting certificate verify locations
Under Windows sometimes you have a problem with curl when you don't have defined a valid and new CA-File.
https://superuser.com/questions/442793/why-cant-curl-properly-verify-a-certificate-on-windows
Here is an example how to download and define a ca file. I don't know if that solves your problem but you can try it. Perhaps then you get a valid connection.