$arr = [
[
"id"=> '1',
"period_id"=> "100",
"police_id"=> 'a01',
"account_name"=> "李四",
"period"=> "测试内容一"
],
[
"id"=> '2',
"period_id"=> '200',
"police_id"=> 'a01',
"account_name"=> "李四",
"period"=> "测试内容二"
],
[
"id"=> '3',
"period_id"=> '300',
"police_id"=> 'a02',
"account_name"=> "张三",
"period"=> "测试内容三"
]
];

$arr = [
[
"id"=> '1、2',
"period_id"=> "100、200",
"police_id"=> 'a01',
"account_name"=> "李四",
"period"=> "测试内容一、测试内容二"
],
[
"id"=> '3',
"period_id"=> '300',
"police_id"=> 'a02',
"account_name"=> "张三",
"period"=> "测试内容三"
]
];
如图所示，如何将原始的二维数组，合并变成想要的二维数组，向各位大佬请教