Separate HTML from Recursive PHP function(s)

I'm trying to separate the HTML elements from the PHP code within Zend Framework 2, but I have no clue how to solve this problem. I'm currently echoing those HTML elements, which does the job. However, there must be a way to separate the HTML from the PHP, rather than echoing the HTML elements.

Currently, I have created a view helper that allows me to generate this tree map for other modules as well, since they will also use this feature, provided that this helper is given a tree map. The categoryTreeMap contains a tree map of category (Doctrine 2 ORM) objects.

This is what I've got so far:

namespace Application\View\Helper;

use Zend\View\Helper\AbstractHelper;

class CategoryTreeMapHelper extends AbstractHelper
{
    public function __invoke($categoryTreeMap)
    {
        echo "<ol class=\"sortable\">";
        foreach ($categoryTreeMap as $category) {
            $this->showCategories($category);
        }
        echo "</ol>";
    }

    public function showCategories($category)
    {
        echo "<li><div>" . $category->name . "</div>";
        if (isset($category->childs)) {
            echo "<ol>";
            foreach ($category->childs as $child_category) {
                $this->showCategories($child_category);
            }
            echo "</ol>";
        }
        echo "</li>";
    }
}

Any suggestions on how to solve this, by separating the HTML from the PHP echo's?

If your helper solely consist of those two methods, you can replicate the functionality in templates by making use of the partial helper

Create a partial for your treemap container

// view/partial-treemap.phtml
<ol class="sortable">
    <?php foreach ($this->categoryTreeMap as $category) :
        echo $this->partial('partial-category', array('category' => $category));
     endforeach; ?>
</ol> 

Create a partial for the recursive part (which calls itself recursively for children)

// view/partial-category.phtml
<li>
    <div><?php echo $category->name; ?></div>
    <?php if (isset($category->childs)) : ?>
    <ol>
        <?php foreach ($category->childs as $child_category) :
            echo $this->partial('partial-category', array('category' => $child_category));
        endforeach; ?>
    </ol>
    <?php endif; ?>
</li>

Then in your controller action view you only need one line

<?php echo $this->partial('partial-treemap', array('categoryTreeMap' => $categoryTreeMap)); ?>