Find nth word/element of string and replace it

Question

I have long unordered list (<ul>) which i need to divide to three columns. I have something like this:

$num = substr_count($ul, '<li>')/3;
$sep = round($num, 0);

...and now i need to find nth element ($sep) of $ul and replace it.

<li> to </ul><ul><li>

EDIT:

My list look like this:

<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li>6</li>
</ul>

...and i want like this:

<ul>
<li>1</li>
<li>2</li>
</ul>
<ul>
<li>3</li>
<li>4</li>
</ul>
<ul>
<li>5</li>
<li>6</li>
</ul>

Answer 1

I ended up with this solution:

$ul = '<ul><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li></ul>';

$doc = new DOMDocument();
$doc->loadHTML('<?xml encoding="UTF-8">' . $ul);
$liList = $doc->getElementsByTagName('li');
$liValues = array();
foreach ($liList as $li) {
    $liValues[] = $li->c14n();
}

$num = count($liValues) / 3;
$size = round($num+1);

$i = 0;
$list = '';

foreach ($liValues as $listItem) {

    if ($i !== 0 && $i % $size === 0) {
        $list.= '</ul><ul>';
    }

    $list.= $listItem;

    $i++;
}

echo '<ul>'.html_entity_decode(str_replace(array('</br>', '</img>'), '', $list)).'</ul>';