Example of how to create a plugin tag with nested loops and prefixed variables.

plugin-tag-example.php

<?php

/* Usage
    
{exp:kaplan:categories
    products="55,56,52,53,54,65"
    state="{url:segment_2_category_id}"
    variable_prefix="category:"}

    {if no_results}{/if}

    {category_list}
        <a href="{site_url}{segment_1}/{url_title}/{category:cat_url_title}">{category:cat_name}</a>
    {/category_list}

{/exp:kaplan:categories}

*/

function categories()
{
    $product_category_ids = $this->EE->TMPL->fetch_param('products', '55,56,52,53,54,65');
    $state_category_id = $this->EE->TMPL->fetch_param('state');
    $prefix = $this->EE->TMPL->fetch_param('variable_prefix', '');

    if ( ! $state_category_id)
    {
        show_error('{exp:kaplan:categories} requires a state id');
    }

    $sql = 'select cp.entry_id, cp.cat_id, c.cat_name, c.cat_url_title
                from exp_category_posts as cp
                join exp_categories as c on cp.cat_id = c.cat_id
                where cp.cat_id in('. $this->EE->db->escape_str($product_category_ids) .') AND exists 
                    (select entry_id from exp_category_posts where cat_id = '. $this->EE->db->escape_str($state_category_id) .' and entry_id = cp.entry_id) 
                group by cat_id';

    $qry = $this->EE->db->query($sql);

    $vars = array();
    $categories = array();
    $i = 1;
    $total_results = $qry->num_rows();

    if ($total_results == 0)
    {
        return $this->EE->TMPL->no_results();
    }

    foreach ($qry->result_array() as $row)
    {
        $prefixed_row = array();

        foreach ($row as $key => $value)
        {
            $prefixed_row[$prefix.$key] = $value;
        }

        $prefixed_row[$prefix.'count'] = $i;
        $prefixed_row[$prefix.'total_results'] = $total_results;

        $categories[] = $prefixed_row;

        $i++;
    }

    $vars[] = array(
        'category_list'         => $categories,
        $prefix.'total_results' => $total_results
    );

    return $this->EE->TMPL->parse_variables($this->EE->TMPL->tagdata, $vars);
}