default.template

server {
    listen 80 default_server;
    
    add_header Content-Security-Policy "frame-ancestors 'self' https://${content_security_policy_url}";
    add_header X-XSS-Protection "1; mode=block";
    root /var/www/prod/public_html;
    include /etc/nginx/fcgiwrap.conf;
    include /etc/nginx/mime.types;
    index index.php index.sec index.html;
    error_page 405 =200 $uri;


    # Create route of /dashboard to the domain name dashboard
    location /dashboard {
        proxy_pass http://$host/dashboard;
    }

    location / {
        try_files $uri $uri/ /index.php?$args;
        # set_real_ip_from ::/0;
        # set_real_ip_from 0.0.0.0/0;
        # real_ip_header X-Forwarded-For;
        fastcgi_param HTTPS $fastcgi_param_https_variable;
        error_page 500 502 503 504  /500.html;
    }
    location ~ .php|.sec$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param SCRIPT_NAME $fastcgi_script_name;
        fastcgi_param HTTPS $fastcgi_param_https_variable;
        # fastcgi_param sitePath "https://$host/";
        include fastcgi_params;
    }
    location = /500.html {
        root /usr/share/nginx/html;
        internal;
    }
}

I get subdomain value which is dynamic and I want to set the SERVER_NAME with the value that sets in HTTP_HOST
Ex: in URL if I hit abc.test.com in browser,$_SERVER[‘SERVER_NAME’] should return the same value or xyz.abc.com, $_SERVER[‘SERVER_NAME’] should be xyz.abc.com.

I tried the following
1.

server {
   server_name ~^(?<subdomain>.+).abc.com$;
}

with this solution, If I print $_SERVER['SERVER_NAME'] in code, it doesnt replace the regex with actual value
2. location ~ .php|.sec$ {
    fastcgi_param SERVER_NAME $http_host
}

This will not set the expected data, SERVER_NAME will be empty

/etc/nginx/fcgiwrap.conf file has
fastcgi_param SERVER_NAME $server_name;

PHP and NGINX are running under docker container

It will be great If I get any inputs on how to set S_SERVER[‘SERVER_NAME’] with HTTP_HOST value

TIA