原理：

1.先查詢表中的記錄總數

2.隨機獲取偏移量為0~總記錄數-1

3.查詢時skip偏移量，再獲取1條記錄

因本人測試環境PHP已升級到7.0以上，mongodb擴展使用支持php7.0以上的擴展，很多方法與php5.6不同。因此代碼必須在php7.0以上運行。如果是php5.6環境，需要修改代碼才能運行。

代碼如下：

function.php

<?php
// 連接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;
}

// 插入數據
function add($conn, $dbname, $collname, $data, $index){

  // 創建索引
  $cmd = array(
    'createIndexes' => $collname,
    'indexes' => array(
      array(
        'name' => 'index',
        'key' => $index,
        'ns' => $dbname.'.'.$collname
      )
    )
  );
  $command = new MongoDBDriverCommand($cmd);
  $conn->executeCommand($dbname, $command);

  // 插入數據
  $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;
}

// 獲取總記錄數
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;
}

// 隨機獲取一條記錄
function randOne($conn, $dbname, $collname){

  // 總記錄數
  $total = getCount($conn, $dbname, $collname);

  // 隨機偏移
  $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;
}

// 對象轉為數組
function objectToArray($obj){
  $arr = is_object($obj) ? get_object_vars($obj) : $obj;
  if(is_array($arr)){
    return array_map(__FUNCTION__, $arr);
  }else{
    return $arr;
  }
}
?>