lib/AmazonSNS.php
public function listSubscriptionsByTopic($topicArn, $nextToken = null) {
		if(empty($topicArn)) {
			throw new InvalidArgumentException('Must supply a TopicARN to show subscriptions to a topic');
		}

		$params = array(
			'TopicArn' => $topicArn
		);

		if(!is_null($nextToken)) {
			$params['NextToken'] = $nextToken;
		}

		$resultXml = $this->_request('ListSubscriptionsByTopic', $params);

		// Get subscriptions
		$subs = $resultXml->ListSubscriptionsByTopicResult->Subscriptions->member;

		$return = ['members' => $this->_processXmlToArray($subs)];

		if(isset($resultXml->ListSubscriptionsByTopicResult->NextToken)) {
			$return['nextToken'] = strval($resultXml->ListSubscriptionsByTopicResult->NextToken);
		}

		return $return;
	}
public function listSubscriptionsByTopic($topicArn, $nextToken = null) {
		if(empty($topicArn)) {
			throw new InvalidArgumentException('Must supply a TopicARN to show subscriptions to a topic');
		}

		$params = array(
			'TopicArn' => $topicArn
		);

		if(!is_null($nextToken)) {
			$params['NextToken'] = $nextToken;
		}

		$resultXml = $this->_request('ListSubscriptionsByTopic', $params);

		// Get subscriptions
		$subs = $resultXml->ListSubscriptionsByTopicResult->Subscriptions->member;

		$return = ['members' => $this->_processXmlToArray($subs)];

		if(isset($resultXml->ListSubscriptionsByTopicResult->NextToken)) {
			$return['nextToken'] = strval($resultXml->ListSubscriptionsByTopicResult->NextToken);
		}

		return $return;
	}
List subscribers to a topic

@link http://docs.amazonwebservices.com/sns/latest/api/API_ListSubscriptionsByTopic.html
@param string $topicArn
@param string|null $nextToken [optional] Token to retrieve next page of results
@return array
@throws InvalidArgumentException