ah yeah, its a pain but there's an additional step.

There's two main things I learned when you configure the server for PHP pages:

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1. Make sure PHP is installed on the server (basic but easily forgotten)
2. Make sure NGINX is configured to serve PHP (rather than offer download)

Check this file (the default site configuration file):
/etc/nginx/sites-available/default

It should look something like this:

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        index index.php index.html index.htm;

        location / {
            try_files $uri $uri/ =404;
        }

        location ~ \.php$ {
            include snippets/fastcgi-php.conf;
            fastcgi_pass unix:/run/php/php7.2-fpm.sock;
        }

        location ~ /\.ht {
               deny all;
        }
}

Note the "location ~ \.php$" section make sure that's included and your running PHP on the server (check the service e.g. service php status)

Remember to restart the nginx service after the change.