Submission hook: What am I doing wrong?
Two things:

Check your argument to fopen for validity, and two, the $form_state in Webform is missing a key. Example:

  $data = array(
    'full_name' =&gt; $form_state['values']['submitted']['full_name'],
    'company' =&gt; $form_state['values']['submitted']['company'],
    'phone' =&gt; $form_state['values']['submitted']['phone'],
    'email_address' =&gt; $form_state['values']['submitted']['email_address'],
    'areas_of_interest' =&gt; array(),
    'comments' =&gt; $form_state['values']['submitted']['comments'],
  );


You are missing ['submitted'].