Description:
------------
It's currently not possible to efficiently run a curl_multi request while simultaneously selecting on other file descriptors. At first glance, this appears to be trivially easy to completely fix. (I'd implement it myself but I'm still getting familiar with PHP's internals.)

According to the cURL documentation at https://curl.se/libcurl/c/curl_multi_wait.html,

----

   CURLMcode curl_multi_wait(CURLM *multi_handle,
                             struct curl_waitfd extra_fds[],
                             unsigned int extra_nfds,
                             int timeout_ms,
                             int *numfds);

   (...Synopsis...)

   The calling application may pass additional curl_waitfd structures which are similar to poll(2)'s
   pollfd structure to be waited on in the same call.

----

It would seem fairly straightforward to simply introduce a 3rd parameter to curl_multi_select():

  curl_multi_select (CurlMultiHandle $multi_handle, float $timeout = 1.0, array $extra_fds = []) : int

The additional streams would then be inserted into extra_fds.

** Main question: are there scenarios in PHP (core) where a Stream-like-object doesn't boil down to a fd? **

FWIW, curl_multi_select() is currently not using extra_fds for anything else, so implementing this shouldn't step on anything's toes. From https://github.com/php/php-src/blob/5b01c4863fe9e4bc2702b2bbf66d292d23001a18/ext/curl/multi.c#L186:

  186  error = curl_multi_wait(mh->multi, /*extra_fds*/NULL, /*extra_nfds*/0,
       (unsigned long) (timeout * 1000.0), &numfds);

My general use case for this is to be able to achieve proper blocking polling on a bunch of streams, when one of those streams is a [set of] cURL requests, for example in the context of async or event loop frameworks.

IMO such libraries can easily bear the load of intelligently handling the "stream_select() vs curl_multi_select()?" question - this problem is endemic to cURL design idiosyncrasies, and cURL's efficiency (and HTTP/3 support, and general batteries-included behavior, etc etc) make it definitively worth it to plumb through the support described here so the PHP userspace can be enabled to fully adapt to cURL's way of doing things.

In my small event loop framework, I've basically had to implement recurring ticks that ultimately make stream_select() timeout every 100000us so I can then curl_multi_select($mh, /*timeout*/0). This is not at all particularly efficient, hence this little feature request.