php数组分组问题

php二维数组按某个字段分组：

<?php

$order_item = Array (  
=> Array ( "order_id" => 1111 ,"item_id" => 1, "pro_id" => 1, "pro_name" => 1111 ,"buy_num" => 5),  
=> Array ( "order_id" => 1111 ,"item_id" => 2 ,"pro_id" => 2 ,"pro_name" => 2222, "buy_num" => 5 ),
=> Array ( "order_id" => 2222 ,"item_id" => 3 ,"pro_id" => 1 ,"pro_name" => 2222, "buy_num" => 1 ),
=> Array ( "order_id" => 2222 ,"item_id" => 4 ,"pro_id" => 2 ,"pro_name" => 2222, "buy_num" => 1 )
);

$items = array();
foreach($order_item as $item) {
    $order_id = $item["order_id"];
    unset($item["order_id"]);
    
    if(!isset($items[$order_id])) {
        $items[$order_id] = array("order_id"=>$order_id, "items"=>array());
    }

    $items[$order_id]["items"][] = $item;
}
// 下面这一步可不要
sort($items);
print_r($items);