How to transfer contents of a string, not a file, directly to client via http download in Drupal?

I'm getting stuck while developing a form in drupal; upon valid submission, I want the form to initiate an http file transfer with 开发者_运维知识库the client, opening a file download prompt with data generated from an in-memory string, not a file.

file_transfer($source, $headers) looked quite promising, but $source is expected to be a URI to a file. Is there a similar function that accepts the contents of string instead of a file URI? In my search through the Drupal API documentation, I have yet to find anything.

I have also tried a (hackish) more manual approach:

header("header statements");
header("more header statements");
echo $string_contents;
exit;

This method of course, interrupts Drupal's form processing, which can't lead to good things in the long term.

Any help would be much appreciated. Thanks!

Something along these lines:

<?php
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"my-file.txt\"");
$data="Hey this is a text file, yay \n";
$data .= "it has stuff in it"; 
echo $data; 
?>

in Drupal there is a function that wraps header() and stores headers statically in an array:

http://api.drupal.org/api/drupal/includes--common.inc/function/drupal_set_header/6

which can then be retrieved with:

http://api.drupal.org/api/drupal/includes--common.inc/function/drupal_get_header/6