尝试从命名空间加载类，是否忘记了“use”语句，用于另一个名称空间

我在代码中使用Cassandra类，这个错误总是显示"试图从名称空间加载类，您是否忘记了另一个名称空间的"use"语句?"这里是我的代码

DefaultController

<?php
namespace Acme'SampleBundle'Controller;
use Symfony'Bundle'FrameworkBundle'Controller'Controller;
use Cassandra;
class DefaultController extends Controller
{
public function indexAction()
{
    $con = new Cassandra'Timestamp();
    echo $con;
}
}

Timestap.php

<?php
/**
* Copyright 2015-2016 DataStax, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
namespace Cassandra;
/**
* A PHP representation of the CQL `timestamp` datatype
*/
final class Timestamp implements Value
{
/**
 * Creates a new timestamp from either unix timestamp and microseconds or
 * from the current time by default.
 *
 * @param int $time Unix timestamp
 * @param int $usec Microseconds
 */
public function __construct($time = null, $usec = null) {}
/**
 * The type of this timestamp.
 *
 * @return Type
 */
public function type() {}
/**
 * Unix timestamp.
 *
 * @return int seconds
 * @see time
 */
public function time() {}
/**
 * Microtime from this timestamp
 *
 * @param bool $get_as_float Whether to get this value as float
 *
 * @return float|string Float or string representation
 * @see microtime
 */
public function microtime($get_as_float = false) {}
/**
 * Converts current timestamp to PHP DateTime.
 *
 * @return 'DateTime PHP representation
 */
public function toDateTime() {}
/**
 * Returns a string representation of this timestamp.
 *
 * @return string timestamp
 */
public function __toString() {}
}

我不知道我的代码出了什么问题，我是使用Cassandra和symfony的新手