Call to undefined function comment_num_replies()

When trying to load a node page with AuthCache enabled, the theme was completely broken and this error was at the bottom of my source code:

<script type="text/javascript">
        Authcache.info = {"Reason":"PHP Error","type":1,"message":"Call to undefined function comment_num_replies()","file":"C:\\web\\htdocs\\sites\\all\\modules\\authcache\\authcache.module","line":564};
</script>

Looking at line 564, it's calling a function (comment_num_replies) which isn't used in Drupal 7, so I removed it and it seems to be working fine now. That's probably not the best way to resolve this, but it was the quickest solution.

Old:
if ($variables['user']->uid && comment_num_replies($variables['comment']->cid) == 0) {

New:
if ($variables['user']->uid) {