OAuth PHP lib-Twitter令牌请求失败

我正试图使用这个PHP库从Twitter请求令牌：http://oauth.googlecode.com/svn/code/php/但它不起作用，我得到"验证oauth签名和令牌失败"。

我的代码：

//http://oauth.googlecode.com/svn/code/php/OAuth.php
require 'OAuth.php';
$key = 'XXXXXX';
$secret = 'XXXXXX';
$consumer = new OAuthConsumer($key, $secret);
$sig_method = new OAuthSignatureMethod_HMAC_SHA1;
$api_endpoint = 'https://api.twitter.com/oauth/request_token';
$parameters = array('oauth_callback'=>'oob');
$req = OAuthRequest::from_consumer_and_token($consumer, null, "POST", $api_endpoint, $parameters);
$sig_method = new OAuthSignatureMethod_HMAC_SHA1();
$req->sign_request($sig_method, $consumer, null);
$ch = curl_init($req->to_url());
curl_exec($ch);
curl_close($ch);