>db.ids.save({name:"user", id:0});

> db.ids.find();
{ "_id" : ObjectId("4c637dbd900f00000000686c"), "name" : "user", "id" : 0 }

>userid = db.ids .findAndModify({update:{$inc:{'id':1}}, query:{"name":"user"}, new:true});
{ "_id" : ObjectId("4c637dbd900f00000000686c"), "name" : "user", "id" : 1 }

>db.user .save({uid:userid.id, username:"dotcoo", password:"dotcoo", info:"http://www.1sucai.cn/ "});
// --------- update 2011-03-27 13:11 ------------------------

>db.user .save({
  uid: db.ids .findAndModify({
    update:{$inc:{'id':1}},
    query:{"name":"user"},
    new:true}).id, //让db.ids集合中的name="user"文档的id值加1并返回充当自增id
  username: "dotcoo",
  password:"dotcoo",
  info:"http://www.1sucai.cn/  "});
// --------- update 2011-03-27 13:11 ------------------------

> db.user.find();
{ "_id" : ObjectId("4c637f79900f00000000686d"), "uid" : 1, "username" : "admin", "password" : "admin" }

<?php
function mid($name, $db){
  $update = array('$inc'=>array("id"=>1));
  $query = array('name'=>$name);
  $command = array(
      'findandmodify'=>'ids', 'update'=>$update,
      'query'=>$query, 'new'=>true, 'upsert'=>true
  );
  $id = $db->command($command);
  return $id['value']['id'];
}
$conn = new Mongo();
$db = $conn->idtest;
$id = mid('user', $db);
$db->user->save(array('uid'=>$id, 'username'=>'kekeles', 'password'=>'kekeles', 'info'=>'http://www.1sucai.cn/ '));
$conn->close();
?>