实现原理如下

    1.先查询表中的记录总数

    2.随机获取偏移量为0~总记录数-1

    3.查询时skip偏移量，再获取1条记录

因本人测试环境php已升级到7.0以上，mongodb扩展使用支持php7.0以上的扩展，很多方法与php5.6不同。因此代码必须在php7.0以上运行。如果是php5.6环境，需要修改代码才能运行。

代码如下：

function.php

<?php
// 连接mongodb
function conn($host, $user, $passwd){
 $server = 'mongodb://'.$user.':'.$passwd.'@'.$host;
 try{
  $conn = new MongoDBDriverManager();
 } catch (MongoDBDriverExceptionConnectionException $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
   )
  )
 );
 $command = new MongoDBDriverCommand($cmd);
 $conn->executeCommand($dbname, $command);

 // 插入数据
 $bulk = new MongoDBDriverBulkWrite();
 $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()
 );
 $command = new MongoDBDriverCommand($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);
 $query = new MongoDBDriverQuery($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;
 }
}
?>