How can I put multiple paths into the same domain in nginx?

I want to connect domain.com/c to /var/www/a/c and domain.com/b to /var/www/b. And I wrote nginx sites-available file like following:

server {
    listen 443 ...

    index index.php index.html;
    server_name domain.com;
    root /var/www/a;

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

    location /b {
        root /var/www/b;
        try_files $uri $uri/ /index.php?$query_string;
    }

    location ~ .php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/run/php/php8.0-fpm.sock;
    }
}

But I see 404 when I enter domain.com/b. I tried using alias instead of root but I got the same result. How can I do?

Answer

It is because your location ~ .php$ { ... } uses your global root /var/www/a;. Here are just two possible solution among many others:

1. Use the nested PHP handler:

   server {
       listen 443 ...
   
       index index.php index.html;
       server_name domain.com;
       root /var/www/a;
   
       location / {
           try_files $uri $uri/ =404;
       }
   
       location ^~ /b/ {
           root /var/www;
           try_files $uri $uri/ /b/index.php?$query_string;
           location ~ .php$ {
               include snippets/fastcgi-php.conf;
               fastcgi_pass unix:/run/php/php8.0-fpm.sock;
           }
       }
   
       location ~ .php$ {
           include snippets/fastcgi-php.conf;
           fastcgi_pass unix:/run/php/php8.0-fpm.sock;
       }
   }
   

   Or if your /var/www/b is the only PHP app, your can just add a root directive to your PHP handler:

   server {
       listen 443 ...
   
       index index.php index.html;
       server_name domain.com;
       root /var/www/a;
   
       location / {
           try_files $uri $uri/ =404;
       }
   
       location /b/ {
           root /var/www;
           try_files $uri $uri/ /b/index.php?$query_string;
       }
   
       location ~ .php$ {
           root /var/www;
           include snippets/fastcgi-php.conf;
           fastcgi_pass unix:/run/php/php8.0-fpm.sock;
       }
   }
   

2. Use the map directive to get your web root from the request URI:

   map $uri $root {
       ~^/b/    /var/www;
       default  /war/www/a;
   }
   server {
       listen 443 ...
   
       index index.php index.html;
       server_name domain.com;
       root $root;
   
       location / {
           try_files $uri $uri/ =404;
       }
   
       location /b/ {
           try_files $uri $uri/ /b/index.php?$query_string;
       }
   
       location ~ .php$ {
           include snippets/fastcgi-php.conf;
           fastcgi_pass unix:/run/php/php8.0-fpm.sock;
       }
   }