How can I export the form generated by Webform into one single .php document?

Hmm, I don't think I've ever seen someone trying to take a form from Drupal and make it separate from Drupal. Because Drupal uses it's own FormsAPI for processing and validating forms, I don't see how something like this would be possible. If you're just looking to duplicate a form, you can use the node_clone module to copy a webform to a new one.

If your determined to get the form "out" of Drupal. You might look at the drupal_form_render function which gets the form array before it's rendered. That would give you the complete form structure in an array form before it's rendered into HTML. Other than that... view source on the page and copy/paste? :)