请教一个简单的递归

$arr = array(
    array(
        'id' => 1,
        'parentid' => 0,
        'title' => 'a'
    ),
    array(
        'id' => 2,
        'parentid' => 1,
        'title' => 'a-1'
    ),
    array(
        'id' => 3,
        'parentid' => 1,
        'title' => 'a-1'
    ),
    array(
        'id' => 4,
        'parentid' => 2,
        'title' => 'a-1-1'
    )  
);

递归转换为数组如下:
$arr = array(
    array(
        'id' => 1,
        'parentid' => 0,
        'title' => 'a',
        'child' => array(
                       array(
                         'id' => 2,
                         'parentid' => 1,
                         'title' => 'a-1',
                         'child' => array(
                                    ......
                                    ) 
                       array(
                         'id' => 3,
                         'parentid' => 2,
                         'title' => 'a-2'
                       )
    ), 
    ......
);
请问要如何写这个递归呢?

回复讨论(解决方案)

function findChildren($list, $p=0){  $r = array();  foreach($list as $id=>$item){    if($item['parentid'] == $p) {      $r[$id] = $item;      if($t = findChildren($list, $item['id']) )        $r[$id]['children'][] = $t;    }  }  return $r;}        $arr = array(    array(        'id' => 1,        'parentid' => 0,        'title' => 'a'    ),    array(        'id' => 2,        'parentid' => 1,        'title' => 'a-1'    ),    array(        'id' => 3,        'parentid' => 1,        'title' => 'a-1'    ),    array(        'id' => 4,        'parentid' => 2,        'title' => 'a-1-1'    )  );print_r(findChildren($arr));