set base URL dynamicaly in Laravel 5

I have two versions of a site one running at on example.com and other at example.com/version2.

In web.php I have

    Route::group(['middleware'=>['webconfig'],'prefix'=>'version2'], function(){
        //Routes for version2 site
    });
// Routes for main site

In webconfig middleware I set database connection for version2 site. Which is working fine but I am also setting base URL for version2 site. which not working url() method returns example.com Here is my webconfig middleware

public function handle($request, Closure $next)
{
    Config::set('database.default', 'mysql_us');
    Config::set('app.url', url('version2'));
    return $next($request);
}

So how can I set base url dynamicaly.

Answer

In your .env file specify APP_URL=http://example.com/version2