Two-legged OAuth Yahoo PHP example - php

Question

I try to make private API requests for Yahoo. I used the code below but it does not work. It always returns the message 'invalid sig'.
Can you help me with a Two-legged OAuth Yahoo PHP example that works?
Thank you very much.
<?php // a super-stripped down 2-leg oauth server/client example
//http://oauth.net/code/
//http://oauth.googlecode.com/svn/code/php/OAuth.php
require 'oauth.php';
$key = 'key';
$secret = 'secret';
$consumer = new OAuthConsumer($key, $secret);
$sig_method = new OAuthSignatureMethod_HMAC_SHA1();
if ($_GET['server']) {
$method = $_SERVER['REQUEST_METHOD'];
$uri = 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['REQUEST_URI'];
$sig = $_GET['oauth_signature'];
$req = new OAuthRequest($method, $uri);
// token is null because we're doing 2-leg
$valid = $sig_method->check_signature($req, $consumer, null, $sig);
if (!$valid) {
die('invalid sig');
}
echo 'orale!';
}
else {
// call this file
$api_endpoint = 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'];
// handle request in 'server' block above
$parameters = array(
'server' => 'true');
// use oauth lib to sign request
$req = OAuthRequest::from_consumer_and_token($consumer, null, "GET", $api_endpoint, $parameters);
$sig_method = new OAuthSignatureMethod_HMAC_SHA1();
$req->sign_request($sig_method, $consumer, null); // note: double entry of token
// get data using signed url
$ch = curl_init($req->to_url());
curl_exec($ch);
curl_close($ch);
}

Answer 1

Do you generate the signature according to the Yahoo specification here?
Or you can use plaintext signature as described here.