FastCGI PHP File not found only when FETCH is used - php

Question

I'm doing a little project to learn Docker - NextJS + PHP backend with Nginx.
Now I got to a stage when I can say it works somehow. When reaching the API via browser adress bar, it gives me the error page - That's right. Nginx not complaining.
BUT when I try to FETCH it, it gives me file not found from the PHP-FPM container and the Nginx complains FastCGI sent in stderr: "Primary script unknown" while reading response header from upstream
The site config is this
upstream next_app {
# NextJS running app port
server nextapp:3000;
}
upstream php_fpm {
# PHP FPM server URI and port
server phpapp:9000;
}
server {
listen 80 default_server;
server_name _;
server_tokens off;
error_log /var/log/nginx/error.log;
access_log /var/log/nginx/access.log;
location ~ ^/api {
root /var/www;
try_files /www/index.php =404;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_pass php_fpm;
}
# proxy pass for NodeJS app
proxy_http_version 1.1;
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
location / {
proxy_pass http://next_app;
}
}
The basic idea was to use PHP api on any request with query starting with /api.
I have a little suspicion on the Next app serving me different Fetch, but I didn't found anything to help it
Any idea how to make it work? Thanks

Answer 1

So, solved now, thanks to #ad7six comments I realized the problem is more accurately described by
"How the hell can I print out a Nginx variable when everything around fails?"
There's two ways - use return and it comes with the response body or
add_header with the always attribute.
add_header X-cgiscript 42 always;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME /var/www/www/index.php;
fastcgi_pass php_fpm;
So now the FPM only executes the index.php of the whole app.
Thanks