PHP实现无限分级

<?php
function tree($arr, $pid=0, $level=0){
    static $list = array();
    foreach($arr as $v){
        if($v['parent_id'] == $pid){
            $v['level'] = $level;
            $list[] = $v;
            tree($arr, $v['cat_id'], $level+1);
        }
    }
    return $list;
}

$arr = array();
$conn = mysql_connect("localhost", "root", "");
mysql_select_db("test");
mysql_query("set names utf8");
$result = mysql_query("select * from category");
$i=0;
while($res = mysql_fetch_assoc($result)){
    $arr[$i]['cat_id'] = $res['cat_id'];
    $arr[$i]['cat_name'] = $res['cat_name'];
    $arr[$i]['parent_id'] = $res['parent_id'];
    $i = $i + 1;
}
$jieshu = tree($arr);
echo "<pre>";
print_r($jieshu);
echo "</pre>";
?>