Here are the objects:

courses
{ "name" : "Biology", "_id" : ObjectId("4b0552b0f0da7d1eb6f126a1") }
students
{
        "name" : "Joe",
        "classes" : [
                {
                        "$ref" : "courses",
                        "$id" : ObjectId("4b0552b0f0da7d1eb6f126a1")
                }
        ],
        "_id" : ObjectId("4b0552e4f0da7d1eb6f126a2")
}

Using the PHP Mongo Class, how do I get all the students that has a biology course?

Thanks