How to use Drupal to encapsule an external site This is the scenario: I have a Drupal 6 site, and another website that doesn't use Drupal. This 'other site' is in the same server, and the same hosting account. I need Drupal to be over the other website with his theme, regions, blocks and sessions/roles/authentication. The other website must conserve its own system of session, authentication, content and their URLs (but like already said, the theme, blocks, regions, authentication, must come from Drupal).

Update: iframe is not an alternative (SEO issues).

Update: This guide Integrating third party applications seems useful, but certainly I do not know how to implement it.
If you've, at least, any control on the second web site you can do it with dynamic includes. It works not so bad with the help of an own module.

The idea its to create a module that do a file_get_content() and return it to Drupal that gonna show the content.

But, for it your second web site should have relative paths between his pages, or you will make the change with a regex.

There an example module, no tested.

function mymodule_menu() {
  $items = array();

  $items['inc/%'] = array(
    'title' =&gt; t("Include"),
    'page callback' =&gt; 'mymodule_include',
    'page arguments' =&gt; array(1)
  );

  return $items;
}

function mymodule_include($path) {
  $http = 'http://example.com/';
  // Maybe check if the page is available 
  // better to use drupal_http_request for that
  $content = file_get_contents($http . $path);

  // Perform any change in the content before to
  // send it for render.
  return $content;
}