Can not obtain a request token twitter Rest API 1.1 - php

Question

since a week I'm trying to manage "Login with twitter" functionality in my website. I can not complete even the first step: 'obtain a request token'. I recieve:
"Failed to validate oauth signature and token"
I'm doing everything like said here: https://dev.twitter.com/docs/auth/implementing-sign-twitter
And creating a signature like here: https://dev.twitter.com/docs/auth/creating-signature
My code:
date_default_timezone_set('UTC');
$oauth_callback = 'http://mydomain.net'; // calls are made from index.php
$url = 'https://api.twitter.com/oauth/request_token';
$oauth_consumer_key = '***';
$oauth_nonce = str_shuffle(trim(base64_encode(time()), '='));
$oauth_signature_method = 'HMAC-SHA1';
$oauth_timestamp = time();
$oauth_token = '***';
$oauth_version = '1.0';
$HTTPMethod = 'POST';
$BaseURL = 'https://api.twitter.com/oauth/request_token';
$consumer_secret = '***';
$access_token_secret = '***';
$params = array(
'oauth_consumer_key' => $oauth_consumer_key,
'oauth_nonce' => $oauth_nonce,
'oauth_signature_method' => $oauth_signature_method,
'oauth_timestamp' => $oauth_timestamp,
'oauth_token' => $oauth_token,
'oauth_version' => $oauth_version,
'oauth_callback' => $oauth_callback
);
function parameter_string (array $params)
{
$temp_array = array();
$parameter_string = '';
while (current($params)) {
$temp_array[rawurlencode(key($params))] = rawurlencode(current($params));
next($params);
}
ksort($temp_array);
foreach ($temp_array as $key => $value) {
$parameter_string .= '&' . $key . '=' . $value;
}
return trim($parameter_string, '&');
}
$parameter_string = parameter_string($params);
function signature_base_string ( $HTTPMethod, $BaseURL, $parameter_string)
{
$signature_base_string = $HTTPMethod . '&' . rawurlencode($BaseURL) . '&' . rawurlencode($parameter_string);
return $signature_base_string;
}
$signature_base_string = signature_base_string ($HTTPMethod, $BaseURL, $parameter_string);
function signing_key($consumer_secret)
{
return rawurlencode($consumer_secret) . '&';
}
$signing_key = signing_key($consumer_secret);
$oauth_signature = base64_encode(hash_hmac('SHA1', $signature_base_string, $signing_key, true));
$header[] = 'Authorization: OAuth '.
'oauth_callback="'.rawurlencode($oauth_callback).'", '.
'oauth_consumer_key="'.rawurlencode($oauth_consumer_key).'", '.
'oauth_nonce="'.rawurlencode($oauth_nonce).'", '.
'oauth_signature="'.rawurlencode($oauth_signature).'", '.
'oauth_signature_method="'.rawurlencode('HMAC-SHA1').'", '.
'oauth_timestamp="'.rawurlencode($oauth_timestamp).'", '.
'oauth_version="'.rawurlencode('1.0').'"';
$options = array(
CURLOPT_URL => $url,
CURLOPT_HEADER => true,
CURLINFO_HEADER_OUT => true,
CURLOPT_HTTPHEADER => $header,
CURLOPT_POST => true,
CURLOPT_RETURNTRANSFER => true
);
$c = curl_init();
$d = curl_setopt_array($c, $options);
$response = curl_exec($c);
echo '<pre>';
print_r($response);
echo '</pre>';
echo '<pre>';
print_r (curl_getinfo($c));
echo '</pre>';
curl_close($c);
The request header looks like this:
POST /oauth/request_token HTTP/1.1
Host: api.twitter.com
Accept: */*
Authorization: OAuth oauth_callback="http%3A%2F%2Fmydomain.net", oauth_consumer_key="***", oauth_nonce="2DHXEIM541CPONJS3UFRQ79G8W0KA6LYZBT", oauth_signature="***", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1379239417", oauth_version="1.0"
Content-Length: 0
Content-Type: application/x-www-form-urlencoded
Please help!