tom_on_wheels You might be able to point it to the flarum PHP file in /var/www/html/flarum (or wherever your Flarum install is). You'll need to be able to provide backup as an argument, however, so it runs php flarum backup. Based on your error message, simply php flarum backup might suffice (you might need the cd part to get to the right directory).
However, I might be misunderstanding your post. When you say call a .php via HTTP, do you mean the file must be publicly available and accessible through the web server? If so, the flarum file isn't (and shouldn't), so that won't work.
In that case, it might be helpful if you share who your hosting provider is.
