Enriching user-entered HTML markup with PHP parsing

<?php
/**
 * Parses the specified markup content for unordered lists, and enriches
 * the list markup with unique identifier classes, 'first' and 'last'
 * classes, 'first-in-row' classes, and a prepended inside element for
 * each list item.
 *
 * @param $content
 *   The markup content to enrich.
 * @param $id_prefix
 *   Each list item is given a class with name 'PREFIX-item-XX'.
 *   Optional.
 * @param $items_per_row
 *   For each Nth element, add a 'first-in-row' class. Optional.
 *   If not set, no 'first-in-row' classes are added.
 * @param $prepend_to_li
 *   The name of an HTML element (e.g. 'span') to prepend inside
 *   each liist item. Optional.
 *
 * @return
 *   Enriched markup content.
 */
function enrich_list_markup($content, $id_prefix = NULL,
$items_per_row = NULL, $prepend_to_li = NULL) {
  // Trim leading and trailing whitespace, DOMDocument doesn't like it.
  $content = preg_replace('/^ */', '', $content);
  $content = preg_replace('/ *$/', '', $content);
  $content = preg_replace('/ *\n */', "\n", $content);

  // Remove newlines from the content, DOMDocument doesn't like them.
  $content = preg_replace('/[\r\n]/', '', $content);

  $doc = new DOMDocument();
  $doc->loadHTML($content);

  foreach ($doc->getElementsByTagName('ul') as $ul_node) {
    $i = 0;
    foreach ($ul_node->childNodes as $li_node) {
      $li_class_list = array();

      if ($id_prefix) {
        $li_class_list[] = $id_prefix . '-item-' . sprintf('%02d', $i+1);
      }

      if (!$i) {
        $li_class_list[] = 'first';
      }
      if ($i == $ul_node->childNodes->length-1) {
        $li_class_list[] = 'last';
      }
      if (!empty($items_per_row) && !($i % $items_per_row)) {
        $li_class_list[] = 'first-in-row';
      }

      $li_node->setAttribute('class', implode(' ', $li_class_list));

      if (!empty($prepend_to_li)) {
        $prepend_el = $doc->createElement($prepend_to_li);
        $li_node->insertBefore($prepend_el, $li_node->firstChild);
      }

      $i++;
    }
  }

  $content = $doc->saveHTML();

  // Manually fix up HTML entity encoding - if there's a better
  // solution for this, let me know.
  $content = str_replace('&acirc;&#128;&#147;', '&ndash;', $content);

  // Manually remove the doctype, html, and body tags that DOMDocument
  // wraps around the text. Apparently, this is the only easy way
  // to fix the problem:
  // http://stackoverflow.com/a/794548
  $content = mb_substr($content, 119, -15);

  return $content;
}
?>

<ul>
  <li>Apples</li>
  <li>Bananas</li>
  <li>Boysenberries</li>
  <li>Peaches</li>
  <li>Lemons</li>
  <li>Grapes</li>
</ul>

<?php
$content = enrich_list_markup($content, 'fruit', 3, 'span');
?>

<ul>
  <li class="fruit-item-01 first first-in-row"><span></span>Apples</li>
  <li class="fruit-item-02"><span></span>Bananas</li>
  <li class="fruit-item-03"><span></span>Boysenberries</li>
  <li class="fruit-item-04 first-in-row"><span></span>Peaches</li>
  <li class="fruit-item-05"><span></span>Lemons</li>
  <li class="fruit-item-06 last"><span></span>Grapes</li>
</ul>

#fruit ul {
  list-style-type: none;
}

#fruit ul li {
  display: block;
  width: 150px;
  padding: 20px 20px 20px 45px;
  float: left;
  margin: 0 0 20px 20px;
  background-color: #bbddfb;
  position: relative;
}

#fruit ul li.first-in-row {
  clear: both;
  margin-left: 0;
}

#fruit ul li span {
  display: block;
  position: absolute;
  left: 20px;
  top: 23px;
  width: 15px;
  height: 15px;
  background-color: #191970;
}

#fruit ul li.first, #fruit ul li.last {
  background-color: #968adc;
}

#fruit ul li.fruit-item-03, #fruit ul li.fruit-item-05 {
  background-color: #7bdca6;
}

#fruit ul li.first span, #fruit ul li.last span {
  background-color: #4b0082;
}

#fruit ul li.fruit-item-03 span, #fruit ul li.fruit-item-05 span {
  background-color: #00611c;
}