Alex Bilbie has passed along a link to a project he's been working on to try to simplify access to MongoDB databases from PHP - his MongoQB library:
A few years ago I released a MongoDB library for the CodeIgniter PHP framework which has become quite popular thanks to it's likeness to CodeIgniter's query builder library. I've just spent the last week travelling and to keep myself occupied I spent some time re-architecting the library's code; removing the framework dependancy, making it Composer friendly and adding a full suite of unit tests. Check out the code on Github - https://github.com/alexbilbie/MongoQB.
The library makes it simpler to do most of the usual CRUD (Create, Read, Update, Delete) operations with your Mongo data. Example code is included in the post. This is a nice lightweight alternative to things like the Doctrine query builder.