How to create the route in symfony 2 which maps to external URL?

You can do this by using one of the Symfony framework controllers although I'm not sure how this would work with parameters:

blog_show:    path: /blog/{slug}    defaults:        _controller: FrameworkBundle:Redirect:urlRedirect        path: "http://example.com/blog"        permanent: true

Note that path: /blog/{slug} grabs the slug directly, but path: "http://example.com/blog/{slug}" doesn't work.

Source: http://symfony.com/doc/current/cookbook/routing/redirect_in_config.html

As of Symfony 2.2 this is possible by adding the host constraint to the routes:

routing.yml

user_homepage:  path: /path/to/whatever  host: "sub.domain.ext"  defaults:     _controller: forExampleAnyNamespaceBundle:Controller:action

There's an official blog post on this issue: http://symfony.com/blog/new-in-symfony-2-2-url-host-support-in-the-routing

The router feature of Symfony doesn't work that way...

I suggest you create a Twig extension for this. Read more about this here:http://symfony.com/doc/current/cookbook/templating/twig_extension.html

You could create a function that works very similar to the regular url() function, so you can migrate as easily as possible.

{{ legacyUrl('blog_post', {slug: 'my-blog-post'}) }}

After you migrated the blog to Symfony, all you need to do is create a route called "blog_post" and change "legacyUrl" to "url".