找不到错误'MongoDB \ Driver \ Manager'

I'm trying to use MongoDB with PHP 7, but I'm getting the error 'MongoDB\Driver\Manager' not found

I used $ composer require mongodb/mongodb to install, and my code is:

use MongoDB\Client;
use MongoDB\Driver\Manager;

class MongoDBSingleton extends AbstractSingleton
{

    private $client;

    protected function __construct()
    {

    //    $manager = new Manager("mongodb://localhost:27017");
    //    $manager = new \MongoDB\Driver\Manager("mongodb://localhost:27017");
    //    die(var_dump($manager));
          $this->client = new Client("mongodb://localhost:27017");
    }
}

I triple checked php.ini where I used extension=mongodb.so and restarted apache