Error Invalid $set :: caused by :: ' emoji' starts with an invalid character for a user variable name

Hello, I’ve tried used php library bulk upsert data value “$$ emoji” has throw error.
I also tried data value “$ emoji” also got unable to upsert.

Below is the code example.

$uri = 'mongodb://localhost:27017/?retryWrites=true&w=majority';
$client = new \MongoDB\Client($uri);
$collection = $client->selectDatabase('test')->test;
$result = $collection->bulkWrite([
    [
        'updateOne' => [
            [
                'unique_id' => '249993de9982967b2950a42ad651e2d7'
            ],
            [
                [
                    '$set' => [
                        'remark' => '$$ emoji'
                    ]
                ]
            ],
            [
                'upsert' => true
            ]
        ]
    ]
]);
db.test.bulkWrite([
   {
       "updateOne": {
           "filter": {"unique_id": "b4f5393a3a7f822eabd3d6eead66e533"},
           "update": [
               { "$set": { "remark": "$$ emoji" } }
           ],
           "upsert": true
       }
   }
]);