如何将MongoDB 聚合查询中的 $substr 操作符转换为 PHP 代码？

$substr: ["<string>", <start>, <length>]

$this->mongo_Collelection->aggregate(    // $where1省略    [        '$project' => [            'date' => ['$substr' => ['time_start', 0, 10]],            'income' => 1,        ],    ],    [        '$group' => [            '_id' => ['date' => '$date'],            'sumincome' => ['$sum' => '$income'],        ],    ],    // $where2省略);