NGINX picking up fastcgi configuration from a different site config

I’m trying to set up a PHPMyadmin instance at db.example.com on an NGINX setup on Ubuntu 20.04 machine.

I have in my sites-enabled directory the config for the following (sub)domains (via symlink from sites-available):

• example.com
• sub.example.com and
• db.example.com

When I initially tried to access the phpmyadmin instance at db.example.com, I was greeted with the error: “No input file specified”, and this answer here provided some hint on a fastcgi misconfiguration.

I’ve changed the fastcgi modular config (conf/php_fastcgi.conf) file referenced in example.com site config (a different host and dir) from:

fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;

to

fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;

This somehow loads the phpmyadmin login page site at db.example.com without any errors.

THE PROBLEM: Here’s where the problem is: Logging in to the phpmyadmin instance now shows

File ./autoload.php missing or not readable. Most likely you did not run Composer to install library files.

It’s likely looking for this path at somewhere else (due to symlinks/routing) as the autoload.php exists in /usr/share/phpmyadmin/ directory. The current db.example.com site config is as mentioned below, in a later section.

I have a few questions in this regard:

1. Why is NGINX picking up a fastcgi config file referenced from a different site config file altogether, in the first place and changing it initially had an effect here?
2. How do I resolve this issue without opening up security issues by letting broad access to files/permissions?

Below are my relevant files: /etc/nginx/nginx.conf: (subset)

# Load configs
include              /etc/nginx/conf.d/*.conf;
include              /etc/nginx/sites-enabled/*;

nginx/sites-available/example.com.conf:

# Default server
server {
    listen       443 ssl http2 default_server;
    server_name  __;
	# SSL
		...
    return       444;
}

server {
    listen              443 ssl http2;
    listen              [::]:443 ssl http2;
    server_name         example.com;
    set                 $base /var/www/example.com;
    root                $base/public;

    # SSL
		...

    # logging
    access_log          /path/to/log.log;
    error_log           /path/to/log.log warn;

    # index.php
    index               index.php;

    # handle .php
    location ~ \.php$ {
        include conf/php_fastcgi.conf;
    }
}

nginx/sites-available/db.example.com.conf:

server {

    listen              443 ssl http2;
    listen              [::]:443 ssl http2;
	
	server_name         db.example.com;
	
	root /var/www/db.example.com/;
    index index.php index.html index.htm;
	
	#logging
	access_log /path/to/log.log;
	error_log /path/to/log.log;

    # SSL
		....

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

    error_page 404 /404.html;
    error_page 500 502 503 504 /50x.html;
    location = /50x.html {
        root /usr/share/nginx/html;
    }

    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
}

A symlink exists for /usr/share/phpmyadmin/ <-> /var/www/db.example.com/ such that phpmyadmin exists as a directory symlink in db.example.com dir and also symlinks exist for all the site configs in sites-available to sites-enabled

Here’s conf directory referenced in the above config:

conf/php_fastcgi.conf:

# 404
try_files                     $fastcgi_script_name =404;

# default fastcgi_params
include                       fastcgi_params;

# fastcgi settings
fastcgi_pass                  unix:/var/run/php/php7.4-fpm.sock;
fastcgi_index                 index.php;
fastcgi_buffers               8 16k;
fastcgi_buffer_size           32k;

# fastcgi params
fastcgi_param DOCUMENT_ROOT   $realpath_root;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param PHP_ADMIN_VALUE "open_basedir=$base/:/usr/lib/php/:/tmp/";

Thanks!

Questions have been referenced in the above post as “THE PROBLEM:”. Thanks!