webform: redirect to modified result page After somebody submits a webform, he should be redirected to the "webform 
result" (which is standard generated, but only accessible by clicking the 
"webform results" tab as an admin).
My module needs to hook into the "webform results", and add some simple custom PHP 
calculations on top of that "webform result" (for example: if two 
radio-elements have a certain value, a warning is displayed on top of that 
result page)


Any ideas ?

cheers
You have do some customization for this, use hook_form_alter(). 


Create a custom module.
Create a result page in that module.
Alter the webform.
Do the calculation.
Sent the data to result page and show.


Example:

function  MODULE_NAME_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (strpos($form_id, 'webform_client_form_') === 0) {
    // webform_client_form_ (change this id with your form id)

   $form['#submit'][] = 'changeform_form_submit';

  }
}

function changeform_form_submit($form, &amp;$form_state) {
  ... your submission logic ...
}