non-blocking-http-client.php

<?php
const HOST = '49.212.25.40';
const PORT = '80';

function createStream() {
    $errorNo = $errorMessage = null;
    $stream = stream_socket_client(HOST . ':' . PORT, $errorNo, $errorMessage, 10);
    if (!$stream) {
        throw new \Exception($errorNo . ':', $errorMessage);
    }

    fwrite($stream, "GET /sleep.php HTTP/1.0\r\n\r\n");

    return $stream;
}

$streams = [
    createStream(),
    createStream(),
    createStream(),
];
$write = $except = [];

while (count($streams) > 0) {
    $read = $streams;
    $ret = stream_select($read, $write, $except, 10);

    if ($ret === false) {
        throw new \Exception('error');
    } else if ($ret === 0) {
        throw new \Exception('could not read');
    } else {
        foreach ($read as $r) {
            $i = array_search($r, $streams);
            echo fread($r, 4096);
            fclose($r);

            unset($streams[$i]);
        }
    }
}

$ php non-blocking-http-client.php
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 20 Mar 2017 06:10:38 GMT
Content-Type: text/html; charset=UTF-8
Connection: close

done
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 20 Mar 2017 06:10:38 GMT
Content-Type: text/html; charset=UTF-8
Connection: close

done
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 20 Mar 2017 06:10:38 GMT
Content-Type: text/html; charset=UTF-8
Connection: close

done

いけそうな気がしてきました！