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);
?>(PHP 5 >= 5.5.0, PHP 7, PHP 8)
curl_multi_setopt — Set a cURL multi option
Sets an option on the given cURL multi handle.
multi_handle
A cURL multi handle returned by curl_multi_init().
option
      One of the CURLMOPT_* constants.
     
value
       The value to be set on option.
       See the description of the
       CURLMOPT_* constants
       for details on the type of values each constant expects.
      
| Version | Description | 
|---|---|
| 8.2.0 | Introduced CURLMOPT_MAX_CONCURRENT_STREAMS. | 
| 8.0.0 | multi_handleexpects a CurlMultiHandle
  instance now; previously, a resource was expected. | 
| 7.1.0 | Introduced CURLMOPT_PUSHFUNCTION. | 
| 7.0.7 | Introduced CURLMOPT_CHUNK_LENGTH_PENALTY_SIZE,CURLMOPT_CONTENT_LENGTH_PENALTY_SIZE,CURLMOPT_MAX_HOST_CONNECTIONS,CURLMOPT_MAX_PIPELINE_LENGTHandCURLMOPT_MAX_TOTAL_CONNECTIONS. | 
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);
?>