$c = [
    ["aid" => 8, "did" => 1],
    ["aid" => 2, "did" => 3],
    ["aid" => 3, "did" => 4],
    ["aid" => 5, "did" => 5],
    ["aid" => 2, "did" => 6],
    ["aid" => 2, "did" => 9],
];


 public function retainLargestDid($array) {
        $maxDids = [];
        $filteredArray = [];

        foreach ($array as $item) {
            $aid = $item['aid'];
            $did = $item['did'];

            // 检查是否已经有这个aid的记录，如果没有或者找到了更大的did，则更新记录
            if (!isset($maxDids[$aid]) || $did > $maxDids[$aid]) {
                $maxDids[$aid] = $did;
            }
        }

        // 遍历原始数组，只保留具有最大did的元素
        foreach ($array as $item) {
            if ($item['did'] === $maxDids[$item['aid']]) {
                $filteredArray[] = $item;
            }
        }

        return $filteredArray;
    }