Requesting CURL_HTTP_VERSION_2 makes curl send an h2c Upgrade request on
plain http URLs, which some servers mishandle. PHP 7.3's built-in server
drops the connection, which failed the curl tests on PHP 7.3. Use
CURL_HTTP_VERSION_2TLS instead, curl's own default since 7.62: https
still negotiates HTTP/2.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>