I get the error in the title when doing something like this:
$zurl = "https://api.zotero.org/something";
require_once 'HTTP/Request2.php';
$r = new HTTP_Request2($zurl);
$r->setMethod(HTTP_Request2::METHOD_GET);
$r->setHeader("Zotero-API-Version", "3");
$r->setHeader("Authorization", "Bearer ".$apiKey);
$response = $r->send();
This article seems to explain it to those who understand. I don't understand, unfortunately.
The article says it is (or might be) a problem with the certificate.
What can I do resolve this problem? (I am on a shared host.)