code: public static function allowedMethod($method)
      {
          if (is_string($method) && ($method = strtoupper(trim($method))) != '') {
              return defined(static::class . '::METHOD_'. $method) ? $method : false;
          }
  
          return false;
      }
docs: Method Allowed
  
  @param string $method
  @return bool|string the method