code: private function codesEqual(string $known, string $given): bool
      {
          if (strlen($given) !== strlen($known)) {
              return false;
          }
  
          $res = 0;
  
          $knownLen = strlen($known);
  
          for ($i = 0; $i < $knownLen; ++$i) {
              $res |= (ord($known[$i]) ^ ord($given[$i]));
          }
  
          return $res === 0;
      }
docs: A constant time code comparison.
  
  @param string $known known code
  @param string $given code received from a user
  
  @return bool
  
  @see http://codereview.stackexchange.com/q/13512/6747