How can i get the absolute url of route with symfony

Using the Routing Component at version 4.0:

<?php
use Symfony\Component\Routing\Generator\UrlGenerator;

UrlGenerator->generate('project_sign_in', [], UrlGenerator::ABSOLUTE_URL);

In Symfony 5.0, if you are using Routing within a service:

namespace App\Service;

use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
.
.
.
private $router;

public function __construct(UrlGeneratorInterface $router)
{
    $this->router = $router;
}

public function foo()
{
    $this->router->generate('bar', [], urlGeneratorInterface::ABSOLUTE_URL);
}

The last facultative parameter has to be true to generate absolute url:

$router->generate('project_sign_in', array(), true);

in twig:

{{ path('project_sign_in', {}, true) }}
{# or #}
{{ url('project_sign_in') }}

in controller:

$this->generateUrl('project_sign_in', array(), true );

EDIT: for symfony 4, see @Michael B. answer
UrlGenerator->generate('project_sign_in', [], UrlGenerator::ABSOLUTE_URL);