如何$push到数组MongoDB PHP驱动程序

我当前有一个包含以下文档的数据库：

{"_id":"5d9bd9429303fc05f0651ff2",

"userID":"2",

"email":"admin@admin.com",

"password":"admin",

"dob":"1990-12-06",

"firstName":"AdminFirst",

"lastName":"AdminLast",

"screenName":"The Admin",

"gender":"m",

"status":"status",

"location":"location",

"visibility":"e",

"friends":[""],

"friendRequests":[""]}

我通过PHP连接到MongoDB，代码如下：

    //connect to the client
$client = new MongoDB\Client("mongodb://localhost:27017");
//Connect to the specific collection
$collection = $client->FBLite->Users;
//Specific user document, stored in session.
$user = $_SESSION['currentDocument'];

我正在尝试$push一个字符串值到我的一个文档的friends数组中。通过PHP实现这一点的正确方法是什么

我试过：

$addFriend = update(array('$push'=> array("friends","4"));)

以及：

---

但是，这两种方法都不起作用，PHP不会抛出任何错误供我阅读。

这是通过以下方法完成的：

$collection->updateOne(
    ['userID' => '2'],
    ['$push' => ['friends' => '55']]
  );

将值55推送到用户ID为“2”的对象

$collection->updateOne(
    ['userID' => '2'],
    ['$push' => ['friends' => '55']]
  );