Description:
------------
When I set my callback in a different function I get a warning. I cannot figure out why. 

If I move the contents of addServerPushCallback() to sendRequest() then I wont have the same issue. 

Test script:
---------------
<?php

class MyHttpClient
{
    private $mh;
    private $curl;

    public function sendRequest()
    {
        if (false === $this->mh = curl_multi_init()) {
            throw new \RuntimeException('Unable to create a new cURL multi handle');
        }

        // FIXME Using a function like this does not work. If we inline the contents of addServerPushCallback(), then we got no problem
        $this->addServerPushCallback();

        $this->curl = curl_init();
        curl_setopt($this->curl, CURLOPT_PROTOCOLS, CURLPROTO_HTTP | CURLPROTO_HTTPS);
        curl_setopt($this->curl, CURLOPT_REDIR_PROTOCOLS, CURLPROTO_HTTP | CURLPROTO_HTTPS);
        curl_setopt($this->curl, CURLOPT_HEADER, false);
        curl_setopt($this->curl, CURLOPT_RETURNTRANSFER, false);
        curl_setopt($this->curl, CURLOPT_FAILONERROR, false);
        curl_setopt($this->curl, CURLOPT_URL, 'https://http2.golang.org/serverpush');
        curl_setopt($this->curl, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0);

        curl_setopt($this->curl, CURLOPT_HEADERFUNCTION, function ($ch, $data) {
            return \strlen($data);
        });

        curl_setopt($this->curl, CURLOPT_WRITEFUNCTION, function ($ch, $data) {
            return \strlen($data);
        });

        curl_multi_add_handle($this->mh, $this->curl);


        $stillRunning = null;
        while (true) {
            do {
                $mrc = curl_multi_exec($this->mh, $stillRunning);
            } while (CURLM_CALL_MULTI_PERFORM === $mrc);

            $info = curl_multi_info_read($this->mh);
            while (false !== $info && $info['msg'] == CURLMSG_DONE) {
                if (CURLMSG_DONE !== $info['msg']) {
                    continue;
                }
                die("Start handle request. ");
            }
        }
    }

    private function addServerPushCallback(): void
    {

        $callback = static function () {
            return CURL_PUSH_OK;
        };

        curl_multi_setopt($this->mh, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);
        curl_multi_setopt($this->mh, CURLMOPT_PUSHFUNCTION, $callback);
    }
}

$buzz = new MyHttpClient();
$buzz->sendRequest();


Expected result:
----------------
Output only "Start handle request."

Actual result:
--------------
Warning: Invalid callback , no array or string given in /usr/src/myapp/test.php on line 40

Warning: curl_multi_exec(): Cannot call the CURLMOPT_PUSHFUNCTION in /usr/src/myapp/test.php on line 40