code: protected function getOauthString()
      {
          $oauth = array_merge($this->getOauthParameters(), array('oauth_signature' => $this->calculateSignature()));
  
          ksort($oauth);
  
          $values = array();
  
          foreach ($oauth as $key => $value) {
              $values[] = $key . '="' . rawurlencode($value) . '"';
          }
  
          $oauth = implode(', ', $values);
  
          unset($values, $key, $value);
  
          return $oauth;
      }
docs: Converting OAuth parameters array to a single string with encoded values
  
  @return string  Single string with encoded values