tree menu

<?php
$data=array(
    array('id'=>1,'url'=>'welcome/index','title'=>'Home','parent_id'=>0),
    array('id'=>2,'url'=>'welcome/about','title'=>'About','parent_id'=>0),
    array('id'=>3,'url'=>'#','title'=>'Product','parent_id'=>0),
    array('id'=>4,'url'=>'welcome/productlist1','title'=>'Product List 1','parent_id'=>3),
    array('id'=>5,'url'=>'welcome/productlist1','title'=>'Product List 1','parent_id'=>4),
    array('id'=>6,'url'=>'welcome/productlist1','title'=>'Product List 1','parent_id'=>5),
    array('id'=>7,'url'=>'welcome/productlist2','title'=>'Product List 2','parent_id'=>5),
    array('id'=>8,'url'=>'welcome/productlist3','title'=>'Product List 3','parent_id'=>5),
    array('id'=>9,'url'=>'welcome/productlist4','title'=>'Product List 4','parent_id'=>5),
    array('id'=>10,'url'=>'welcome/productlist5','title'=>'Product List 5','parent_id'=>5),
    array('id'=>11,'url'=>'welcome/productlist6','title'=>'Product List 6','parent_id'=>5),
    array('id'=>12,'url'=>'welcome/productlist7','title'=>'Product List 7','parent_id'=>5),
    array('id'=>13,'url'=>'welcome/productlist2','title'=>'Product List 2','parent_id'=>4),
    array('id'=>14,'url'=>'welcome/productlist3','title'=>'Product List 3','parent_id'=>4),
    array('id'=>15,'url'=>'welcome/productlist4','title'=>'Product List 4','parent_id'=>4),
    array('id'=>16,'url'=>'welcome/productlist5','title'=>'Product List 5','parent_id'=>4),
    array('id'=>17,'url'=>'welcome/productlist6','title'=>'Product List 6','parent_id'=>4),
    array('id'=>18,'url'=>'welcome/productlist7','title'=>'Product List 7','parent_id'=>4),
    array('id'=>19,'url'=>'welcome/productlist2','title'=>'Product List 2','parent_id'=>3),
    array('id'=>20,'url'=>'welcome/productlist1','title'=>'Product List 1','parent_id'=>19),
    array('id'=>21,'url'=>'welcome/productlist2','title'=>'Product List 2','parent_id'=>19),
    array('id'=>22,'url'=>'welcome/productlist7','title'=>'Product List 7','parent_id'=>19),
    array('id'=>23,'url'=>'welcome/productlist3','title'=>'Product List 3','parent_id'=>3),
    array('id'=>24,'url'=>'welcome/productlist4','title'=>'Product List 4','parent_id'=>3),
    array('id'=>25,'url'=>'welcome/productlist1','title'=>'Product List 1','parent_id'=>24),
    array('id'=>26,'url'=>'welcome/productlist2','title'=>'Product List 2','parent_id'=>24),
    array('id'=>27,'url'=>'welcome/productlist7','title'=>'Product List 7','parent_id'=>24),
    array('id'=>28,'url'=>'welcome/productlist5','title'=>'Product List 5','parent_id'=>3),
    array('id'=>29,'url'=>'welcome/productlist6','title'=>'Product List 6','parent_id'=>3),
    array('id'=>30,'url'=>'welcome/productlist7','title'=>'Product List 7','parent_id'=>3),
    array('id'=>31,'url'=>'welcome/about','title'=>'Article','parent_id'=>0),
    array('id'=>32,'url'=>'welcome/contact','title'=>'Contact','parent_id'=>0)
);

function GenerateDataArray($arr, $parent = 0, $level=0)
{
    $pages = Array();
    foreach($arr as $page)
    {
        if($page['parent_id'] == $parent)
        {
            $page['level']= isset($page['sub'])? $level+1:$level;
            $page['sub'] = isset($page['sub']) ? $page['sub'] : GenerateDataArray($arr, $page['id'], $level+1);
            $pages[] = $page;
        }
    }
    return $pages;
}

echo array_reduce(
   $menu = GenerateDataArray($data), $cb = function($m, $i) use (&$cb) {
      $m .= "<li><a href='{$i['url']}.php'>{$i['title']}</a>";
      if (isset($i['sub']) && !empty($i['sub']))
         $m .= array_reduce($i['sub'], $cb, "<ul>") . "</ul>";
      $m .= "</li>";
      return $m;
   }, "<ul>"
) . "</ul>";