Symfony2 - How to perform an external Request

https://github.com/sensio/SensioBuzzBundle seems to be what you are looking for.

It implements the Kris Wallsmith buzz library to perform HTTP requests.

I'll let you read the doc on the github page, usage is pretty basic:

$buzz = $this->container->get('buzz');

$response = $buzz->get('http://google.com');

echo $response->getContent();

I'd suggest using CURL:

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'www.someapi.com?param1=A&param2=B');
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-type: application/json')); // Assuming you're requesting JSON
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

$response = curl_exec($ch);

// If using JSON...
$data = json_decode($response);

Note: The php on your web server must have the php5-curl library installed.

Assuming the API request is returning JSON data, this page may be useful.

This doesn't use any code that is specific to Symfony2. There may well be a bundle that can simplify this process for you, but if there is I don't know about it.

Best client that I know is: http://docs.guzzlephp.org/en/latest/

There is already bundle that integrates it into Symfony2 project: https://github.com/8p/GuzzleBundle

$client   = $this->get('guzzle.client');

// send an asynchronous request.
$request = $client->createRequest('GET', 'http://httpbin.org', ['future' => true]);
// callback
$client->send($request)->then(function ($response) {
    echo 'I completed! ' . $response;
});

// optional parameters
$response = $client->get('http://httpbin.org/get', [
    'headers' => ['X-Foo-Header' => 'value'],
    'query'   => ['foo' => 'bar']
]);
$code = $response->getStatusCode();
$body = $response->getBody();

// json response
$response = $client->get('http://httpbin.org/get');
$json = $response->json();

// extra methods
$response = $client->delete('http://httpbin.org/delete');
$response = $client->head('http://httpbin.org/get');
$response = $client->options('http://httpbin.org/get');
$response = $client->patch('http://httpbin.org/patch');
$response = $client->post('http://httpbin.org/post');
$response = $client->put('http://httpbin.org/put');

More info can be found on: http://docs.guzzlephp.org/en/latest/index.html

Symfony doesn't have a built-in service for this, but this is a perfect opportunity to create your own, using the dependency injection framework. What you can do here is write a service to manage the external call. Let's call the service "http".

First, write a class with a performRequest() method:

namespace MyBundle\Service;

class Http
{    
    public function performRequest($siteUrl)
    {
        // Code to make the external request goes here
        // ...probably using cUrl
    }
}

Register it as a service in app/config/config.yml:

services:
    http:
        class: MyBundle\Service\Http

Now your controller has access to a service called "http". Symfony manages a single instance of this class in the "container", and you can access it via $this->get("http"):

class MyController
{
    $response = $this->get("http")->performRequest("www.something.com");

    ...
}