How to excute $function operator with mongodb and php?

Working with Data
,
#1

Is there something help me write $function operator with mongodb and php? I try some ways but it doesn’t work:

array_push($pipeline, ['$set' => ['todos' => [
                        '$function' => [
                            'body' => 'function(todos) {
                                return todos;
                            }',
                            'args' => ['todos'],
                            'lang' => 'js',
                        ]
                ]]]);

==> todos array is converts to string “todo”
or

array_push($pipeline, ['$set' => ['todos' => [
                        '$function' => [
                            'body' => function($todos) {
                                return $todos;
                            },
                            'args' => ['todos'],
                            'lang' => 'js',
                        ]
                ]]]);

it doesn’t work.