If you want to enable both HTTP/1.1 pipelining and HTTP/2 multiplexing...
<?php
curl_multi_setopt($mh, CURLMOPT_PIPELINING, 3);
?>
or
<?php
curl_multi_setopt($mh, CURLMOPT_PIPELINING, CURLPIPE_HTTP1 | CURLPIPE_MULTIPLEX);
?>
If you want to enable both HTTP/1.1 pipelining and HTTP/2 multiplexing...
<?php
curl_multi_setopt($mh, CURLMOPT_PIPELINING, 3);
?>
or
<?php
curl_multi_setopt($mh, CURLMOPT_PIPELINING, CURLPIPE_HTTP1 | CURLPIPE_MULTIPLEX);
?>