MongoDB using only map without reduce in PHP - php

Question

In mongoDB i have two collection users and posts following this structure:
Posts
{
_id: ObjectId(""),
subject: "some post",
content: "here is the content",
user_id: "4351"
}
Users
{
user_id: "4351",
name: "John Marks",
picURL: "http://...",
aboutme: "historian of the future"
}
needing to get the posts in array with name.
db.posts.find().map(function(newPost){
newPost.name = db.users.findOne({user_id: newPost.user_id}).name;
return (newPost);
})
I wrote this code and it's work in mongoshell well returning this result:
{
_id: ObjectId(""),
subject: "some post",
content: "here is the content",
user_id: "4351",
name: "John Marks"
}
but i could not apply in php. You can't just simple get the output of the map function. It requires reduce function and output collection for the returning value.
Edit:
$map = new MongoCode('
function(newPost) {
newPost.username = db.users.findOne({user_id: newPost.user_id}).name;
return newPost;
}
');
post = $app->mongo->command(array(
"mapreduce" => "posts",
"map" => $map,
"reduce" => '',
"out" => array("inline" => 1)
));
var_dump($post);
This code must be work but accessing another collection in map function via 'db' is forbidden after mongo 2.4 release. That's why i changed my approach. Instead of using map/reduce, handled with php. Added posts user_ids to array and get the users information with following code.
$userInf = $app->mongo->selectCollection("users")->find(
array('user_id' => array('$in' => $user_ids)),
array("_id" => 0, "user_id" => 1, "name" => 1, "picURL" => 1)
);