php数组分组问题

<?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);