client.php

<?php

require __DIR__ . "/vendor/autoload.php";

use Http\Client\Curl\Client;
use Http\Message\MessageFactory\DiactorosMessageFactory;
use Http\Message\StreamFactory\DiactorosStreamFactory;
use Http\Discovery\HttpAsyncClientDiscovery;
use Http\Discovery\HttpClientDiscovery;
use Psr\Http\Message\ResponseInterface;

$client = new Client(new DiactorosMessageFactory(), new DiactorosStreamFactory());
$factory = new DiactorosMessageFactory();

$foo = $factory->createRequest("GET", "http://localhost:8081/foo");
$bar = $factory->createRequest("GET", "http://localhost:8081/bar");
$pop = $factory->createRequest("GET", "http://localhost:8081/pop");

$promises[] = $client
    ->sendAsyncRequest($foo)
    ->then(function (ResponseInterface $response) {
        print date("H:s") . " got foo\n";
        return $response;
    });

$promises[] = $client
    ->sendAsyncRequest($bar)
    ->then(function (ResponseInterface $response) {
        print date("H:s") . " got bar\n";
        return $response;
    });

$promises[] = $client
    ->sendAsyncRequest($pop)
    ->then(function (ResponseInterface $response) {
        print date("H:s") . " got pop\n";
        return $response;
    });

try {
    array_walk($promises, function($promise) {
        $promise->wait();
    });
} catch (\Exception $exception) {
    print $exception->getMessage() . "\n\n";
}

server.php

<?php

require __DIR__ . "/vendor/autoload.php";

$app = new \Slim\App;

$app->get("/foo", function($request, $response, $args) {
    sleep(3);
    print "foo";
});

$app->get("/bar", function($request, $response, $args) {
    sleep(3);
    print "bar";
});

$app->get("/pop", function($request, $response, $args) {
    sleep(3);
    print "pop";
});


$app->run();