NGINX for pixel serving & host detection springboard

Example index.php file:
<?php
//if ($_SERVER['REMOTE_HOST'] != "_") {
//echo $_SERVER['SERVER_NAME'] . " blocked.";
// } else {
//echo "hi.";
//}
echo $_SERVER['HTTP_HOST'];
?>


Example nginx config:
(empty.gif is a tiny 1x1px image)

server {
        listen 80 default_server;
        listen [::]:80 default_server;
        root /var/www/html;
        index index.html index.htm index.php;
        autoindex off;
        server_name _;
        error_page 404 /empty.gif;
        error_page 415 /empty.gif;
        error_page 410 /empty.gif;
        location / {
                expires max;
                try_files $uri $uri/ =404;
}
location ~ \.php$ {
       include snippets/fastcgi-php.conf;
#
#       # With php-fpm (or other unix sockets):
       fastcgi_pass unix:/run/php/php7.2-fpm.sock;
       #       # With php-cgi (or other tcp sockets):
#       fastcgi_pass 127.0.0.1:9000;
}
        location ~ /\.ht {
                deny all;
        }

}