In the long documentation, it's easy to miss the fact that CURLOPT_POSTFIELDS will set the Content-Type to "multipart/form-data" (instead of the usual "application/x-www-form-urlencoded") IFF you supply an array (instead of a query string)!
Some servers will return weird errors (like "SSL read: error:00000000:lib(0):func(0):reason(0), errno 104") for the wrong Content-Type, and you may waste many hours of time trying to figure out why!