code: public function getSignature($baseString, array $params) {
          // Remove oauth_signature if present
          // Ref: Spec: 9.1.1 ("The oauth_signature parameter MUST be excluded.")
          unset($params['oauth_signature']);
  
          if ($this->signature_method === 'HMAC-SHA1') {
              $result = $this->sign_HMAC_SHA1($baseString);
          }
          else if ($this->signature_method == 'RSA-SHA1') {
              $result = $this->sign_RSA_SHA1($baseString);
          }
          else if ($this->signature_method == 'PLAINTEXT') {
              $result = $this->sign_PLAINTEXT($baseString);
          }
          else {
              throw new ArtaxServiceException('Unknown signature method: '
                  . $this->signature_method);
          }
  
          return base64_encode($result);
      }
docs: Calculate signature for request
  
  @param Request    $request Request to generate a signature for
  @param array      $params  Oauth parameters.
  
  @return string
  
  @throws \RuntimeException