Category Ul-Li Tree structure For Menu Items

function category_navigation($pid,$level)
{
global $res;
global $catUlList;
$res[0] = "-Root-";
$blank = "";

for($i=0; $i< $level; $i++)
$blank .= "\t";



$parents = $this->find('all' , array('conditions' => array('Category.parent_id' => $pid)));

if(!empty($parents))
{
$catUlList .= "\n$blank

";
}

$level++;



foreach($parents as $value)
{
unset($value['ParentCategory']);
unset($value['Product']);

$catName = $value['Category']['name'];
$catUrlName = str_replace(" ","_",$value['Category']['name']);


$catUlList .= "\n$blank
• ";
  
  $catUlList .= "";
  $catUlList .= $catName;
  $catUlList .= "";
  
  //$catUlList .= $html->link($catName, SITE_URL."products/".$value['Category']['id'].":".$catUrlName);
  
  $this->category_navigation($value['Category']['id'],$level);
  $catUlList .= "\n$blank
";
}

if(!empty($parents))
{
$catUlList .= "\n$blank

";
}


$fp = fopen("category_ul_list.php","w");
fwrite($fp,$catUlList);

return $catUlList;
}