title: mongo db find function in php
body: <p>I am building a social network in mongo db.
  Following is the structure of document in the groups collection. It represents each social group present in the system. Users are a part of this group which is represented by the 'members' property which is an array if json objects. </p>
  
  <pre><code>{
    "group_name":"my group",
    "members": [
      {
        "user_id": ObjectId("4e29341986ba75dc14000000"),
        "joined": ISODate("2011-07-26T11:46:49.0Z")
      },
      {
        "user_id": ObjectId("4e2ea94286ba75f81500000e"),
        "joined": ISODate("2011-07-26T11:47:55.0Z")
      },
      {
        "user_id": ObjectId("4e2eaa0786ba75e815000003"),
        "joined": ISODate("2011-07-26T11:55:22.0Z")
      },
      {
        "user_id": ObjectId("4e2eab7f86ba75ec1500000a"),
        "joined": ISODate("2011-07-26T11:57:44.0Z")
      },
      {
        "user_id": ObjectId("4e2eac3586ba75dc15000000"),
        "joined": ISODate("2011-07-26T12:00:57.0Z")
      },
      {
        "user_id": ObjectId("4e2eacae86ba75dc15000004"),
        "joined": ISODate("2011-07-26T12:02:43.0Z")
      },
      {
        "user_id": ObjectId("4e2eadbb86ba75ec1500000c"),
        "joined": ISODate("2011-07-26T12:07:01.0Z")
      }
    ]
  </code></pre>
  
  <p>The problem I am facing is how do I check if a user belongs to a certain group.
  This is what I came up with but it doesn't seem to work.
  Please help.</p>
  
  <pre><code>    $criteria = array(
          '_id' =&gt; new MongoId($group_id),
          'members' =&gt; array('user_id' =&gt; new MongoId($user_id))
      );
  
  
     return $collection-&gt;find($criteria);
  </code></pre>