Why would findOne($query) work on a collection and find($query) not using PHP's Mongo driver?

I'm using PHP to talk with a Mongo use findOne($query) on a db.collection it returns a result as I would expect. But changing that command to find() returns nothing.

From the shell, if you use db.collection.find() it returns all documents. Can anyone explain why the PHP driver's find() wouldn't return results even when findOne() does while using the exact same query?

UPDATE: Here's the code.

find() (doesn't work):

  $db = $connection->selectDB( $database );
  $returned_collection = $db->selectCollection( $collection );
  $cursor = $returned_collection->find( $query );

);

find() debug output:

query: array(1) {
  ["user_id"]=>
  string(13) "4d03d13b71676"
}
1292099894 > mongo_wrapper.class.php > returned_collection: events.votes 
db: object(MongoDB)#41 (2) {
  ["w"]=>
  int(1)
  ["wtimeout"]=>
  int(10000)
}
cursor: object(MongoCursor)#4开发者_Python百科3 (0) {
}

findOne() (works):

  $db = $connection->selectDB( $database );
  $returned_collection = $db->selectCollection( $collection );
  $cursor = $returned_collection->findOne( $query );

findOne() debug output:

query: array(1) {
  ["user_id"]=>
  string(13) "4d03d13b71676"
}
1292099906 > mongo_wrapper.class.php > returned_collection: events.votes 
db: object(MongoDB)#7862 (2) {
  ["w"]=>
  int(1)
  ["wtimeout"]=>
  int(10000)
}
cursor: array(7) {
  ["_id"]=>
  object(MongoId)#7849 (1) {
    ["$id"]=>
    string(24) "4d03d842d0645afaab4e92f6"
  }
  ["user_id"]=>
  string(13) "4d03d13b71676"
  ["timestamp"]=>
  int(1292095809)
  ["context"]=>
  string(3) "ms3"
  ["uri"]=>
  string(120) "http://feeds.marketwatch.com/~r/marketwatch/podcasts/MarketwatchStupidInvestmentOfTheWeek/~3/3H-tMQLS9AA/siotw103009.mp3"
  ["type"]=>
  string(8) "category"
  ["vote"]=>
  int(-1)
}

Both use this debug code:

    if($debug->enabled) {
        echo time() . " > mongo_wrapper.class.php > returned_collection: $returned_collection \n";
        if($debug->dump) {
            echo "db: ";
            var_dump( $db );
            echo "cursor: ";
            var_dump( $cursor );
        }
    }

<?php

$connection = new Mongo();

$db = $connection->database;

$collection = $db->collection;

echo '<pre>';

print_r($collection->findOne());

$cursor = $collection->find();
foreach ($cursor as $id => $value) {
    echo "$id: ";
    print_r($value);
}

echo '</pre>';

?>