code: public static function getSignature($method)
      {
          switch ($method) {
              case 'HMAC-SHA1':
                  return new Signature\HMACSHA1();
                  break;
  
              case 'RSA-SHA1':
                  return new Signature\RSASHA1();
                  break;
  
              case 'PLAINTEXT':
                  return new Signature\PLAINTEXT();
                  break;
  
              default:
                  throw new RuntimeException('Invalid signature method');
                  break;
          }
      }
docs: Returns the signature object based on the $method throws an exception if
  the method is not supported
  
  @param string $method
  @return \PSX\Oauth\SignatureAbstract