example.php

<?php

use Symfony\Component\HttpFoundation\Response;

class MyTest extends FakeHttpTestCase
{
    public function testFakeResponseString()
    {
        $this->mockResponse('GET', '/', 'foo');

        $content = file_get_contents('http://127.0.0.1:8000/');
        
        $this->assertSame('foo', $content);
    }
    
    public function testFakeResponse()
    {
        $this->mockResponse('GET', '/', new Response('foo'));

        $content = file_get_contents('http://127.0.0.1:8000/');
        
        $this->assertSame('foo', $content);
    }
    
    public function testFakeCallback()
    {
        $this->mockResponse(
            'GET',
            '/',
            static function (\Symfony\Component\HttpFoundation\Request $request) {
                if ('foo' === $request->headers->get('X-Auth')) {
                    return new Response('{}', 200, ['Content-Type' => 'application/json']);
                } else {
                    return new Response('Forbidden', 403, ['Content-Type' => 'application/json']);
                }
            }
        );

        $content = file_get_contents('http://127.0.0.1:8000/');
        
        $this->assertSame('Forbidden', $content);
    }
}

FakeHttpTestCase.php

<?php

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

abstract class FakeHttpTestCase extends \PHPUnit_Framework_TestCase
{
    const APP_URL = 'http://127.0.0.1:8000';
    
    /**
     * @var array
     */
    private $fakedRequests = array();

    protected function tearDown()
    {
        if (!empty($this->fakedRequests)) {
            file_get_contents(self::APP_URL.'/_fakes', false, stream_context_create(array('http' => array('method'  => 'DELETE'))));
        }

    }

    /**
     * @param string          $method
     * @param string          $uri
     * @param string|Response $response
     */
    protected function mockResponse($method, $uri, $response)
    {
        if (is_callable($response)) {
            $serializer = new \SuperClosure\Serializer();
            $response = $serializer->serialize($response);
        } else {
            $response = serialize($response);
        }

        $this->fakedRequests[$uri][$method] = $response;
    }
    
    /**
     * Call this after faking all responses, but before making requests in your test 
     */
    protected function fakeResponses()
    {
        if (!empty($this->fakedRequests)) {
            $context = stream_context_create(array('http' =>
                array(
                    'method'  => 'POST',
                    'header'  => 'Content-type: application/json',
                    'content' => json_encode($this->fakedRequests)
                )
            ));

            file_get_contents(self::APP_URL.'/_fakes', false, $context);
        }
    }
}

index.php

<?php

require_once __DIR__.'/../vendor/autoload.php';

use Silex\Application;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use SuperClosure\Serializer;

$app = new Application(['debug' => true]);

$app->before(function (Request $request, Application $app) {
    if (file_exists(sys_get_temp_dir().'/fake-responses.json')) {
        $responses = json_decode(file_get_contents(sys_get_temp_dir().'/fake-responses.json'), true);

        if (!$responses) {
            return new Response('Faked responses file is corrupted', 500);
        }

        foreach ($responses as $uri => $methods) {
            foreach ($methods as $method => $serializedResponse) {
                $method = strtolower($method);
                if (0 === strpos($serializedResponse, 'C:32:"SuperClosure\SerializableClosure')) {
                    $serializer = new Serializer();

                    $callback = $serializer->unserialize($serializedResponse);

                    $responseCallback = $callback;
                } else {
                    $responseCallback = function () use ($serializedResponse) {
                        return unserialize($serializedResponse);
                    };
                }

                $app->$method($uri, $responseCallback);
            }
        }

        $app->flush();
    }
}, Application::EARLY_EVENT);

$app->post('/_fakes', function (Request $request) {
    file_put_contents(sys_get_temp_dir().'/fake-responses.json', $request->getContent());

    return new Response(null, 201);
});

$app->delete('/_fakes', function (Request $request) {
    if (file_exists(sys_get_temp_dir().'/fake-responses.json')) {
        unlink(sys_get_temp_dir().'/fake-responses.json');
    }

    return new Response(null, 202);
});

$app->run();