How to get the output of another Horde function back as a string, even if that function itself outputs directly to the browser:
$html = Util::bufferOutput(array($formnew, 'renderActive'), $RENDERER, $vars, 'manageforum.php', 'post');
Which is the same as the following function being called:
$formnew->renderActive($RENDERER, $vars, 'manageforum.php', 'post');
... but we now have its output in the $html variable.