getting $host from $_SERVER

As part of something else, I have a very small site running on a raspberry pi (apache2.4.38, php8.1.15) and normally it works perfectly but sometimes the log file shows a crash on config/constants. I tracked it down to the following:

PHP Code:

$host = $_SERVER['HTTP_HOST'];
   $http_https = isset($_SERVER['HTTPS']) ? "https://" : "http://";
   $baseURL = $http_https . $host;
   define('BASE_URL',$baseURL); 


I have set htaccess to force https. Sometimes when the site is accessed, don't know why, HTTP_HOST is missing from $_SERVER. The HTTPS key is also missing. Maybe in order to make Constants.php bullet-proof it would be good to have a fallback eg

PHP Code:

if(array_key_exists('HTTP_HOST',$_SERVER)) $host = $_SERVER['HTTP_HOST'];
   else $host=$_SERVER['SERVER_NAME']; 


Bill