code: public static function kernel()
      {
          if($os = static::os(true)){
              $os = explode(' ', $os);
              if('/' == DIRECTORY_SEPARATOR){
                  $kernel = $os[2];
              }else{
                  $kernel = $os[1];
              }
          }else{
              $kernel = '';
          }
          
          return $kernel;
      }
docs: Server kernel
  @return string