code: private function validateBase64($value)
      {
          $isNotValid = base64_decode($value, true) === false;
          if ($isNotValid) {
              $this->throwInvalidArgumentMustBe("valid base_64");
          }
      }
docs: Allowed characters: A-Z, a-z, 0-9, +, /, =
  
  @param $value