It's a little unclear what this actually says the problem is. Reading the PHP code, it seems to suggest that even if the transfers run OK, curl_error can return an error string saying it failed to resolve the host name. Is that correct?
Do not rely on the contents of the buffer unless an error code was returned.
Oh wow I think you are correct. It looks like that the return code is correctly HTTP/2 200.
Is there anything that changed though in 7.63 and 7.64 which could make the buffer be filled with a message like Could not resolve host: www.example.com even though the response from the server was returned correctly?
What makes you even suspect this is related to #3446 ?
Sorry for confusing here, I was just searching through the git issues and found a very suspect issue regarding multi connections in curl. Realizing now that the response actually worked, I agree that it has nothing to do with it.
When a transfer is done, the resolver thread will be brought down. That
could accidentally generate an error message in the error buffer even
though this is not an error situationand the transfer would still return
OK. An application that still reads the error buffer could find a
"Could not resolve host: [host name]" message there and get confused.
Reported-by: Michael Schmid
CURLOPT_ERRORBUFFER doc says "Do not rely on the contents of the
buffer unless an error code was returned." 
Prior to this change the error buffer was returned even if no error had
occurred, and that buffer may contain incorrect information in such a
Basically copying this over from https://stackoverflow.com/questions/54771917/could-not-resolve-host-error-with-curl-multi-on-curl-version-7-64
I'm not the author of the StackOverflow question, but I have the exact same issue.
I did this
This will upgrade curl to 7.64.0 which has the issue:
I expected the following
This will keep curl on 7.63.0 which does not have the issue
Could this maybe have to do with #3446 ?
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