code: public static function getProtocol($url = false)
      {
          if ($url) {
              return (preg_match('/^https/', $url)) ? 'https' : 'http';
          }
  
          $protocol = strtolower($_SERVER['SERVER_PROTOCOL']);
          $protocol = substr($protocol, 0, strpos($protocol, '/'));
  
          $ssl = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on');
  
          return ($ssl) ? $protocol . 's' : $protocol;
      }
docs: Get protocol from current or passed URL.
  
  @param string $url
  
  @return string → http|https