mongo 没有 ping函数 所以需要参照PDO的写法,   findOne   Throws MongoConnectionException if it cannot reach the database.

// mongo instance
function getMongoCollection($col)
{
    static $mongoClient, $collection = [];

    try {

        // init mongoClient
        if (!$mongoClient) {
            $mongoClient = mongoClientInit();
        }

        // check mongoClient
        if (!is_object($mongoClient) || !method_exists($mongoClient, 'selectCollection')) {
            throw new \MongoConnectionException('reconnect mongo failed');
        }

        // init mongoCollection
        if (!isset($collection[$col])) {
            $collection[$col] = mongoCollectionInit($mongoClient, $col);
        }

        // ping mongo
        $collection[$col] = mongoPing($collection[$col], $col);

        // check mongoPing again
        if (!is_object($collection[$col])) {
            throw new \MongoConnectionException('reconnect mongo failed');
        }

    } catch (\MongoConnectionException $e) {
        error_log($e->getMessage(), 3, ERROR_LOG);
    }


    return $collection[$col] ?: null;
}

function mongoPing($collection, $col)
{
    try {

        // check collection
        if (!$collection || !method_exists($collection, 'findOne')) {
            throw new \MongoConnectionException('mongo connect failed');
        }

        // Throws MongoConnectionException if it cannot reach the database.
        $collection->findOne([], ['_id' => 1]);

    } catch (\MongoConnectionException $e) {

        // log
        error_log('mongo connect failed and try to reconnect', 3, ERROR_LOG);

        // init MongoClient
        $mongoClient = mongoClientInit();

        // init MongoCollection
        if (is_object($mongoClient) && method_exists($mongoClient, 'selectCollection')) {
            $collection = mongoCollectionInit($mongoClient, $col);
        } else {
            $collection = null;
        }

    }

    return $collection;
}


function mongoClientInit()
{

    $config = get_config('mongo');

    // reconnect mongo 3 times
    $mongoClient = null;
    $i = 0;
    while ($i < 3 && !$mongoClient) {
        try {
            $mongoClient = new MongoClient($config['dsn']);
        } catch (\MongoConnectionException $e) {
            $i++;
        }
    }

    return $mongoClient;
}

function mongoCollectionInit($mongoClient, $col)
{
    $config = get_config('mongo');

    // get MongoCollection object
    $i = 0;
    $collection = null;
    while ($i < 3 && !$collection) {
        try {

            $collection = $mongoClient->selectCollection($config['db'], $col);

        } catch (\Exception $e) {
            $i++;
        }
    }

    return $collection;
}