mongodb，update方法无效

我的数据里有
{ "_id" : ObjectId("509b645910ade1be6c000012"), "name" : "aaa", "age" : "21" }
{ "_id" : ObjectId("509b645910ade1be6c000013"), "name" : "bbb", "age" : "20" }
{ "_id" : ObjectId("509b645910ade1be6c000014"), "name" : "ccc", "age" : "25" }
{ "_id" : ObjectId("509b645d10ade1be6c000015"), "name" : "aaa", "age" : "21" }
{ "_id" : ObjectId("509b645d10ade1be6c000016"), "name" : "bbb", "age" : "20" }
{ "_id" : ObjectId("509b645d10ade1be6c000017"), "name" : "ccc", "age" : "25" }
{ "_id" : ObjectId("509b645e10ade1be6c000018"), "name" : "aaa", "age" : "21" }
{ "_id" : ObjectId("509b645e10ade1be6c000019"), "name" : "bbb", "age" : "20" }
{ "_id" : ObjectId("509b645e10ade1be6c00001a"), "name" : "ccc", "age" : "25" }
然后使用PHP修改数据
`$conn = new Mongo("mongodb://root:111111@127.0.0.1/local");
$db = $conn->local; //选择test数据库
$collection = $db->phptest; //若此表不存在则会自行创建
$newdata = array("$set" => array("age" => "888811111"));
$collection->update(array('name' => 'aaa'), $newdata,false,true);`
执行PHP之后数据没有改变，这是什么原因呢？