title: MongoDB-PHP: JOIN-like query
body: <p>Here are the objects:</p>
  
  <pre><code>courses
  { "name" : "Biology", "_id" : ObjectId("4b0552b0f0da7d1eb6f126a1") }
  students
  {
          "name" : "Joe",
          "classes" : [
                  {
                          "$ref" : "courses",
                          "$id" : ObjectId("4b0552b0f0da7d1eb6f126a1")
                  }
          ],
          "_id" : ObjectId("4b0552e4f0da7d1eb6f126a2")
  }
  </code></pre>
  
  <p>Using the PHP Mongo Class, how do I get all the students that has a biology course?</p>
  
  <p>Thanks</p>