src/Sonic/Helper.php
public function url_for($file_name)
    {
        // Check if file exists
        try {

            // Try to read file, if none exists then return null
            $this->app['filesystem']->read($this->app['settings']['sonic.content_dir'] . '/' . $file_name);

            // Create URL and return
            $clean_file_name = str_replace(
                array_merge(array('index'), $this->dot_extensions()),
                '',
                $file_name
            );

            return trim($this->app['settings']['site.site_root'] . $clean_file_name, '/') . '/';
        }
        catch (\Exception $e) {
            $this->app['router']->error($e);
        }
        return null;
    }
public function url_for($file_name)
    {
        // Check if file exists
        try {

            // Try to read file, if none exists then return null
            $this->app['filesystem']->read($this->app['settings']['sonic.content_dir'] . '/' . $file_name);

            // Create URL and return
            $clean_file_name = str_replace(
                array_merge(array('index'), $this->dot_extensions()),
                '',
                $file_name
            );

            return trim($this->app['settings']['site.site_root'] . $clean_file_name, '/') . '/';
        }
        catch (\Exception $e) {
            $this->app['router']->error($e);
        }
        return null;
    }
Returns a fully-qualified URL for a given filename in the 'content' directory

@param  string $file_name
@return string|null