Close open HTML tags, e.g. if you allow HTML in your comments. Not quite as robust as running it through tidy, but tidy is not always available.

function close_dangling_tags($html){
  #put all opened tags into an array
  preg_match_all("#<([a-z]+)( .*)?(?!/)>#iU",$html,$result);
  $openedtags=$result[1];

  #put all closed tags into an array
  preg_match_all("#</([a-z]+)>#iU",$html,$result);
  $closedtags=$result[1];
  $len_opened = count($openedtags);
  # all tags are closed
  if(count($closedtags) == $len_opened){
    return $html;
  }

  $openedtags = array_reverse($openedtags);
  # close tags
  for($i=0;$i < $len_opened;$i++) {
    if (!in_array($openedtags[$i],$closedtags)){
      $html .= '</'.$openedtags[$i].'>';
    } else {
      unset($closedtags[array_search($openedtags[$i],$closedtags)]);
    }
  }
  return $html;
}