menu.php

<?php
function get_menu_html($items, $root_id = 0) {
    $html = array();

    foreach ($items as $item)
        $children[$item['parent_id']][] = $item;

    // loop will be false if the root has no children (i.e., an empty menu!)
    $loop = !empty($children[$root_id]);

    // initializing $parent as the root
    $parent = $root_id;
    $parent_stack = array();

    // HTML wrapper for the menu (open)
    $html[] = '<ul>';

    while ($loop && ( ( $option = each($children[$parent]) ) || ( $parent > $root_id ) )) {
        if ($option === false) {
            $parent = array_pop($parent_stack);

            // HTML for menu item containing childrens (close)
            $html[] = str_repeat("\t", ( count($parent_stack) + 1 ) * 2) . '</ul>';
            $html[] = str_repeat("\t", ( count($parent_stack) + 1 ) * 2 - 1) . '</li>';
        } elseif (!empty($children[$option['value']['id']])) {
            $tab = str_repeat("\t", ( count($parent_stack) + 1 ) * 2 - 1);

            // HTML for menu item containing childrens (open)
            $html[] = sprintf(
                '%1$s<li><a href="%2$s">%3$s</a>', $tab, // %1$s = tabulation
                $option['value']['href'], // %2$s = link (URL)
                $option['value']['title']   // %3$s = title
            );
            $html[] = $tab . "\t" . '<ul class="submenu">';

            array_push($parent_stack, $option['value']['parent_id']);
            $parent = $option['value']['id'];
        }
        else
        // HTML for menu item with no children (aka "leaf") 
            $html[] = sprintf(
                '%1$s<li><a href="%2$s">%3$s</a></li>', str_repeat("\t", ( count($parent_stack) + 1 ) * 2 - 1), // %1$s = tabulation
                $option['value']['href'], // %2$s = link (URL)
                $option['value']['title']   // %3$s = title
            );
    }

    // HTML wrapper for the menu (close)
    $html[] = '</ul>';
    return implode("\n", $html);
}