Change root directory based on request url – Nginx

We have a website where we are using a Laravel for the part available to the end-user, and an older legacy codebase for the admin panel.

Currently root is the public folder for the currently deployed version.

The website folder structure looks something like this:

admin/
  |____index.php <-- desired entrypoint for admin-related requests
app/

public/
  |____index.php <-- main entrypoint for website

resources/

routes/

So, when someone wants to access the admin panel they go to example.com/admin

Here’s the current nginx configs file we are using.

example.com

server {
    server_name example.com;
    root /var/www/example.com/current/public;

    add_header X-Frame-Options "SAMEORIGIN";
    add_header X-Content-Type-Options "nosniff";

    index index.php;

    charset utf-8;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    location /admin {
        proxy_pass http://localhost:3000;
    }

    error_page 404 /index.php;
    
    location ~ .php$ {
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
        include fastcgi_params;
    }
    
    location ~ /.(?!well-known).* {
        deny all;
    }
}

example.com/admin

server {
    listen 3000;
    server_name example.com;
    root /var/www/example.com/current/admin;

    add_header X-Frame-Options "SAMEORIGIN";
    add_header X-Content-Type-Options "nosniff";

    index index.php;

    charset utf-8;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    location = /favicon.ico { access_log off; log_not_found off; }
    location = /robots.txt  { access_log off; log_not_found off; }

    error_page 404 /index.php;

    location ~ .php$ {
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
        include fastcgi_params;
    }

    location ~ /.(?!well-known).* {
        deny all;
    }
}

I’m not very well versed in nginx server configuration and would love som assistance on this.

EDIT 1

Config file using map:

EDIT 2

Updated the config below with the changes made to make it work.

map $uri $siteroot {
    # This didn't work, per the accepted answer.
    # ^/admin  /var/www/example.com/current/admin;

    # This works great!
    ^/admin   /var/www/example.com/current;
    default   /var/www/example.com/current/public;
}

server {
    server_name example.com;
    root $siteroot;

    add_header X-Frame-Options "SAMEORIGIN";
    add_header X-Content-Type-Options "nosniff";

    index index.php;

    charset utf-8;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    error_page 404 /index.php;

    location ~ .php$ {
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
        include fastcgi_params;
    }

    location ~ /.(?!well-known).* {
        deny all;
    }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

server {
    if ($host = example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    listen 80;
    server_name example.com;
    return 404; # managed by Certbot
}