Put an instance of this class into the array: http://php.net/manual/en/class.mongodb-bson-utcdatetime.php
You need to use the BSON objects in the PHP arrays: http://php.net/manual/en/book.bson.php
Those get serialized properly before passed to Mongo.
$test = array(
"_id" => 123,
"name" => "This is an example",
"date_created" => new MongoDB\BSON\UTCDateTime(time() * 1000)
);
$collection->insert($test);
The time() is multiplied by 1000, because the constructor wants the milliseconds elapsed since the unix epoch, and time() returns the seconds elapsed since the unix epoch. See: http://php.net/manual/en/mongodb-bson-utcdatetime.construct.php
UPDATE:
To retrive the date/time in ISO format, just convert the UTCDateTime object fist to DateTime:
$date_created = $test["date_created"];
$iso_date = $date_created->toDateTime()->format("Y-m-d H:i:s");