gistfile1.php

<?php

function cache_invalidation_node_presave($node) {
  if(!empty($node->promote)) {//target front page content

    //Clear drupal cache
    cache_clear_all('http://cachedemo.local/', 'cache_page');

    //Clear varnish cache
    httprl_request( 'http://cachedemo.local/', array('blocking' => FALSE, 'method' => 'PURGE'));
    httprl_send_request();
  }
}