Drupal 7: hook_webform_submission_presave() to subscribe a user to a Simplenews newsletter with a conditional component.

d7_webform_simplenews_conditional_subscription.php

<?php
/*
* Implementation of hook_webform_submission_presave()
*
* For a particular Webform node, use the submitted
* value of a component to determine whether or not 
* the submitter will be subscribed to a Simplenews
* newsletter. 
*
* Subscribe the user to the Simplenews newsletter
* using the value of the email component on the form.
*
* In this example, the Webform node id will be 42
*/
function yourmodulename_webform_submission_presave($node, &$submission){
  // Choose the webform ID
  if($submission->nid == 42){
    /*
    * Here I check my controlling component.
    * I used a checkbox component to let users
    * check a box if they wanted to subscribe.
    * 
    * If the value is NULL, do nothing -- they
    * don't want to subscribe.
    */
    if($submission->data[4][0] != NULL){
      /*
      * The user wants to subscribe, use the value of 
      * this Webform's email component.
      * In my form, I decided to have a hidden field that I would use
      * to subscribe users.
      */ 
      // Set the email component value as the hidden field's value too.
      $submission->data[5][0] = $submission->data[3][0];
      // Create the user array.
      $user = array(
        'snid' => '',
        'activated' => '1',
        'mail' => $submission->data[3][0],
        'uid' => 0,
        'language' => 'English',
      );
      // Create the subscription array.
      $subscribe = array(
        'snid' => '',
        'tid' => 1,
        'status' => 1,
        'timestamp' => time(),
        'source' => 'Your Module Name',
      );
      // Create an object out of the user array.
      $subscriber = (object) $user;
      if(simplenews_user_is_subscribed($subscriber->mail, 1)){
        // Do nothing, user is already subscribed 
      }else{
        // Subscribe them.
        simplenews_subscriber_save($subscriber);
        simplenews_subscribe_user($subscriber->mail, 1, FALSE, 'Your Module Name');
      }
    }  
  }
}
?>