Rightsignature API始终返回“无效的OAuth请求”

I have created API keys after created an account with rightsignature. I downloaded the PHP rightsignature library from this RS-PHP-Embedded-Sample - After analysis the library, I just created a sample file and written the below code for testing.

<?php
require_once("config/common.inc.php");    

$rightsignature->getRequestToken();

$rightsignature->getAccessToken();

But I'm always getting getAccessToken() method response as 'Invalid OAuth Request' even thought I'm using valid oAuth Customer Key, oAuth Customer Secret keys. I have no idea what I'm doing wrong here. Please help me to resolve this issue.

we can find some other PHP libraries related to rightsignature on this url - api_documentation_php

But nothing helped me.

Thanks in advance.