如何在mongodb中随机获取1条记录？

<?php
// 连接mongodb
function conn($host, $user, $passwd){
 $server = 'mongodb://'.$user.':'.$passwd.'@'.$host;
 try{
 $conn = new MongoDB\Driver\Manager();
 } catch (MongoDB\Driver\Exception\ConnectionException $e){
 throw new ErrorException('Unable to connect to db server. Error:' . $e->getMessage(), 31);
 }
 return $conn;
}

// 插入数据
function add($conn, $dbname, $collname, $data, $index){

 // 创建索引
 $cmd = array(
 'createIndexes' => $collname,
 'indexes' => array(
  array(
  'name' => 'index',
  'key' => $index,
  'ns' => $dbname.'.'.$collname
  )
 )
 );
 $command = new MongoDB\Driver\Command($cmd);
 $conn->executeCommand($dbname, $command);

 // 插入数据
 $bulk = new MongoDB\Driver\BulkWrite();
 $inserted = 0;

 if($data){
 foreach($data as $k=>$v){
  $bulk->insert($v);
 }
 $result = $conn->executeBulkWrite($dbname.'.'.$collname, $bulk);
 $inserted = $result->getInsertedCount();
 }

 return $inserted;
}

// 获取总记录数
function getCount($conn, $dbname, $collname){
 $cmd = array(
 'count' => $collname,
 'query' => array()
 );
 $command = new MongoDB\Driver\Command($cmd);
 $result = $conn->executeCommand($dbname, $command);
 $response = current($result->toArray());
 if($response->ok==1){
 return $response->n;
 }
 return 0;
}

// 随机获取一条记录
function randOne($conn, $dbname, $collname){

 // 总记录数
 $total = getCount($conn, $dbname, $collname);

 // 随机偏移
 $skip = mt_rand(0, $total-1);

 $filter = array();
 $options = array('skip'=>$skip, 'limit'=>1);
 $query = new MongoDB\Driver\Query($filter, $options);
 $cursor = $conn->executeQuery($dbname.'.'.$collname, $query);

 $result = array();
 if($cursor){
 foreach($cursor as $v){
  $v = objectToArray($v);
  unset($v['_id']);
  $result[] = $v;
 }
 }

 return $result? $result[0] : $result;
}

// 对象转为数组
function objectToArray($obj){
 $arr = is_object($obj) ? get_object_vars($obj) : $obj;
 if(is_array($arr)){
 return array_map(__FUNCTION__, $arr);
 }else{
 return $arr;
 }
}
?>

<?php
require('function.php');

// 连接mongodb
$conn = conn('localhost','testdb','root','123456');

// 插入50条数据记录
$data = array();

// 索引
$index = array('user'=>true);

for($i=0; $i<50; $i++){
 $data[] = array(
 'user' => 'test_user_'.str_pad($i, 4, '0', STR_PAD_LEFT)
 );
}

$inserted = add($conn, 'testdb', 'user', $data, $index);
echo '成功插入'.$inserted.'条测试记录数<br><br>';

// 随机获取一条记录,抽5次
echo '随机获取一条记录，抽5次<br>';
$result = array();
for($i=0; $i<5; $i++){
 $result[] = randOne($conn, 'testdb', 'user');
}

echo '<pre>';
print_r($result);
echo '</pre>';
?>

Array
(
 [0] => Array
 (
  [user] => test_user_0017
 )

 [1] => Array
 (
  [user] => test_user_0026
 )

 [2] => Array
 (
  [user] => test_user_0004
 )

 [3] => Array
 (
  [user] => test_user_0043
 )

 [4] => Array
 (
  [user] => test_user_0023
 )

)

use testdb

db.createUser( 
 { 
 "user":"root", 
 "pwd":"123456", 
 "roles":[{"role" : "readWrite", "db":"testdb"}] 
 } 
) 

db.auth( 
 { 
 "user":"root", 
 "pwd":"123456" 
 } 
)