php 父子 数据结构 转成 多维数组

$arr = array (
array('id' => 1 , 'fid' => 0 , 'content' => 'dsadasdasdadasda'),
array('id' => 2 , 'fid' => 1 , 'content' => 'dasdawqe31231'),
array('id' => 3 , 'fid' => 1 , 'content' => 'dasda64564564564wqe31231'),
array('id' => 4 , 'fid' => 0 , 'content' => '765765757575'),
array('id' => 5 , 'fid' => 4 , 'content' => '97897898797897'),
array('id' => 6 , 'fid' => 0 , 'content' => '97897898797897'),
array('id' => 7 , 'fid' => 6 , 'content' => '97897898797897'),
array('id' => 8 , 'fid' => 7 , 'content' => '97897898797897'),
);

function bar($arr , $flag = 0 , $res = array() )
{
foreach ($arr as $k => $v)
{
if($v['fid'] == $flag )
{
$res[$v['id']] = $v;
bar($arr , $v['id'] , &$res[$v['id']]['sub'] );
}
}
}

bar($arr , 0 , &$res);
print_r($res);