Symfony 2.7 has a new absolute_url which can be used to generate the absolute url.
http://symfony.com/blog/new-in-symfony-2-7-the-new-asset-component#template-function-changes
It will work on those both cases or a path string:
<a href="https://stackoverflow.com/questions/17049712/{{ absolute_url(path("route_name', {'param' : value})) }}">A link</a>
and for assets:
<img src="https://stackoverflow.com/questions/17049712/{{ absolute_url(asset("bundle/myname/img/image.gif')) }}" alt="Title"/>
Or for any string path
<img src="https://stackoverflow.com/questions/17049712/{{ absolute_url("my/absolute/path') }}" alt="Title"/>
on those tree cases you will end up with an absolute URL like
http://www.example.com/my/absolute/path