code: public function requestToken(Url $url, $consumerKey, $consumerSecret, $method = 'HMAC-SHA1', $callback = null)
      {
          $values = array(
              'oauth_consumer_key'     => $consumerKey,
              'oauth_signature_method' => $method,
              'oauth_timestamp'        => self::getTimestamp(),
              'oauth_nonce'            => self::getNonce(),
              'oauth_version'          => self::getVersion(),
          );
  
          // if we have an callback add them to the request
          if (!empty($callback)) {
              $values['oauth_callback'] = $callback;
          } else {
              $values['oauth_callback'] = 'oob';
          }
  
          // build the base string
          $requestMethod = 'POST';
          $params        = array_merge($values, $url->getParameters());
          $baseString    = self::buildBasestring($requestMethod, $url, $params);
  
          // get the signature object
          $signature = self::getSignature($method);
  
          // generate the signature
          $values['oauth_signature'] = $signature->build($baseString, $consumerSecret);
  
          // request unauthorized token
          $request  = new PostRequest($url, array(
              'Authorization' => 'OAuth realm="psx", ' . self::buildAuthString($values),
              'User-Agent'    => __CLASS__,
          ));
          $response = $this->client->request($request);
  
          // parse the response
          return $this->importer->import(new Response(), (string) $response->getBody());
      }
docs: Requests a new "request token" from the $url using the consumer key and
  secret. The $url must be valid request token endpoint. Returns an array
  with all key values pairs from the response i.e.
  <code>
  $response = $oauth->requestToken(...);
  
  $token       = $response->getToken();
  $tokenSecret = $response->getTokenSecret();
  </code>
  
  @see http://tools.ietf.org/html/rfc5849#section-2.1
  @param \PSX\Uri\Url $url
  @param string $consumerKey
  @param string $consumerSecret
  @param string $method
  @param string $callback
  @return \PSX\Oauth\Data\Response