Build recursive nested menu from array

I would index all the menu items (pages) by id first. In the second loop associate menu items with their parents and keep track of top level items.

One of the ways I would do it (Render method is just an example):

$sqlGetDropdownPages = "SELECT * FROM `cms_staticPages` ORDER BY `sortOrder` ASC";
$qryGetDropdownPages = mysql_query($sqlGetDropdownPages) or die(mysql_error());

//indexing pages by id
while ($pageRow = mysql_fetch_assoc($qryGetDropdownPages)) {
    $menuItems[$pageRow['id']] = new MenuItem($pageRow);
}

//associating pages with parents
foreach ($menuItems as $menuItem) {
    if ($menuItem->hasParent()) {
        $menuItem->addToParent($menuItems);
    } else {
        $topLevel[] = $menuItem;
    }
}

//$topLevel is an array of menuItems without parents
$render = '<ul>';
foreach ($topLevel as $menuItem) {
    $render .= $menuItem->render(true);
}
$render .= '</ul>';

MenuItem class would look something like this

class MenuItem {

    /**
     * @var array
     */
    private $data;

    /**
     * @var MenuItem[]
     */
    private $children = array();

    /**
     * @var MenuItem
     */
    private $parent;

    /**
     * @param array $data
     */
    public function __construct($data) {
        $this->data = $data;
    }

    /**
     * Finds the parent in the collection and adds itself to it
     * @param MenuItem[] $menuItems
     */
    public function addToParent($menuItems) {
        if (isset($menuItems[$this->data['parent_id']])) {
            $this->parent = $menuItems[$this->data['parent_id']];
            $this->parent->addChild($this);
        }
    }

    /**
     * @return bool
     */
    public function hasParent() {
        return empty($this->data['parent_id']);
    }

    /**
     * @param MenuItem $child
     */
    public function addChild(MenuItem $child) {
        $this->children[] = $child;
    }

    /**
     * Renders the menu with ul li
     * @param bool $withChildren
     * @return string
     */
    public function render($withChildren = false) {
        $render = '<li>' . $this->data['identifier'] . '</li>';

        if ($withChildren) {
            $render .= '<ul>';
            foreach ($this->children as $child) {
                $render .= $child->render(true);
            }
            $render .= '</ul>';
        }

        return $render;
    }

}