php二维数组相同id的数量相加

$arr = array(
array(
'user_id' => 100,
'goods_id' => 10,
'number' => 1,
),
array(
'user_id' => 100,
'goods_id' => 10,
'number' => 2,
),
array(
'user_id' => 100,
'goods_id' => 10,
'number' => 2,
),
array(
'user_id' => 100,
'goods_id' => 10,
'number' => 2,
),
);

$item=array();
foreach($arr as $k=>$v){
if(!isset($item[$v['user_id']])){
$item[$v['user_id']]=$v;
}else{
$item[$v['user_id']]['number']+=$v['number'];
}
}
print_r($item);