title: MongoDB hint doesn't seem to work
body: <p>This is a slightly weird one.
  I'm iterating through quite a large database, using a query that checks if certain elements exist. running this query in database without hint takes about 137 seconds, but with hint it goes down to 0 (which is obv preferable). When using it via php</p>
  
  <pre><code>$collection-&gt;find($params)-&gt;hint(array("responses" =&gt; 1))-&gt;count();
  </code></pre>
  
  <p>and profiling the queries I'm getting this:</p>
  
  <blockquote>
    <p>{ "ts" : ISODate("2012-05-08T10:11:24.682Z"), "op" : "command", "ns" :
    "ey_gsat.$cmd", "command" : { "count" : "survey_answers", "query" : {
    "responses.93" : { "$exists" : true }, "responses.96" : { "$exists" :
    true }, "responses.99" : { "$exists" : true }, "responses.102" : {
    "$exists" : true }, "responses.105" : { "$exists" : true },
    "responses.108" : { "$exists" : true }, "responses.111" : { "$exists"
    : true }, "responses.114" : { "$exists" : true }, "responses.117" : {
    "$exists" : true }, "responses.120" : { "$exists" : true },
    "responses.1" : { "$exists" : true } } }, "ntoreturn" : 1,
    "responseLength" : 48, "millis" : 137, "client" : "127.0.0.1", "user"
    : "" }</p>
  </blockquote>
  
  <p>There is no sign of the query actually using an index when run via php. I have quite a few queries like this running on this site, so the 137 milliseconds all add up, hence the reason why I would like to optimise them. any ideas what might be wrong?</p>