title: How to use _id field with $lt operator
body: <p>It seems as if <code>less than</code> construct does not work, as I expect it. I try to  implement it like this:</p>
  
  <pre><code>...
  while( $cursor-&gt;hasNext() ){
      $doc = $cursor-&gt;getNext();
      $last_id = $doc['_id']-&gt;{'$id'};
  }
  $condition = array('_id' =&gt; array('$lt' =&gt; $last_id));
  $new_cursor = $collection-&gt;find($condition)-&gt;sort(array('_id' =&gt; -1))-&gt;limit(250);
  </code></pre>
  
  <p>I also tried '$gt', but in both situations I get empty result-sets. NB. $last_id is a valid id, which probably needs some conversion before I use it in condition block.</p>