PHP array manipulation in tree structure

Here you are my solution:

<?php
$data=Array
(
    0 => Array
        (
            'id' => 1,
            'sub_id' => 1,
            'title' => 'TITLE 1',
            'url' => 'www.title1.com',
        ),

    1 => Array
        (
            'id' => 4,
            'sub_id' => 4,
            'title' => 'TITLE 2',
            'url' => 'www.title2.com',
        ),

    2 => Array
        (
            'id' => 2,
            'sub_id' => 1,
            'title' => 'SUB TITLE 1-1',
            'url' => 'www.subtitle1.com',
        ),

    3 => Array
        (
            'id' => 3,
            'sub_id' => 2,
            'title' => 'SUB TITLE 1-2',
            'url' => 'www.subtitle2.com',
        ),

    4 => Array
        (
            'id' => 5,
            'sub_id' => 4,
            'title' => 'SUB TITLE 2-1',
            'url' => 'www.subtitle2.com',
        ),

);

function find_index(&$arr, $ind){
    if(empty($arr))
        return false;
    else {
        foreach($arr as &$aa){
            if($aa['id']==$ind['sub_id']) {
                $key=count($aa['sub_ar']);
                $aa['sub_ar'][$key]=$ind;
                $aa['sub_ar'][$key]['sub_ar']=array();
                return true;
            }
            else{
                $res=find_index($aa['sub_ar'],$ind);
                if($res)
                    return true;
            }
        }
        return false;
    }
}

$res=array();
foreach($data as $key=>$dat){
    if($dat['sub_id']==$dat['id']){
        $res[$key]=$dat;
        $res[$key]['sub_ar']=array();
    }
    else {
        $ret=find_index($res,$dat);
    }
}
?>
<pre><?php print_r($res); ?></pre>