Why does php only work in some directories of my Nginx webserver?

I had my Nginx web server set up just the way I needed it to work - but now I need it to work in a sub directory of my server - and, perhaps more importantly, on php which is embedded in html.

My server is all based on Docker containers, with a separate container for php, for nginx and for mariadb.

The directory structure (outside of the containers, where all the served files live) is as follows…

htdocs
|--Common     (where my CSS, JS, etc is)
|--Resources  (where image and video assets are)
|--labs       (where I experiment)
|  |--ExperimentSite
|--blog       (where I blog)
|--forum      (Forums)
|--family

I've tried .php files in root (that works file), in WordPress (also fine) and Vanilla (fine too) - but when I try to load a .php file in /Labs/ExperimentSite it downloads the .php rather than running it. Also, I can't run .php embedded within HTML.

I feel sure that the problem must be with my site .conf file, but even after much experimentation I can't get it to work (although I have worked out how to break my website quite a lot!)

server {
        listen 0.0.0.0:8080;
        server_name  mywebsitename_placeholder.com www.mywebsitename_placeholder.com;
        return 301 https://$server_name$request_uri;
}

server {
        listen 0.0.0.0:8443;
        server_name  mywebsitename_placeholder.com www.mywebsitename_placeholder.com;
        ssl_certificate     bitnami/certs/www.mywebsitename_placeholder.com.crt;
        ssl_certificate_key bitnami/certs/www.mywebsitename_placeholder.com.key;
        server_tokens off;
        
        error_log "/opt/bitnami/nginx/logs/mywebsitename_placeholder-error.log";
        access_log  "/opt/bitnami/nginx/logs/mywebsitename_placeholder-access.log";

        error_page 403 /forbidden.html;
        error_page 404 /lost.html;

        location / {
                try_files $uri $uri/main.html;
        }

        location ~ \.php$ {
        # fastcgi_pass [PHP_FPM_LINK_NAME]:9000;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                fastcgi_pass phpfpm-mywebsitename:9000;
                fastcgi_index index.php;
                include fastcgi.conf;
                root /app;
                error_page 401 /badpassword.php;
                error_page 403 /forbidden.html;
                error_page 404 /lost.html;
                fastcgi_intercept_errors on;
        }

        location ~ \.html$ {
                root   /app/mywebsitename/htdocs;
        }
 
        location ~* \.(js|css|png|jpg|jpeg|gif|ico|dmg|zip|svg|xml)$ {
                expires max;
                log_not_found off;
                root   /app/mywebsitename/htdocs;
        }
        
        location /blog {
                try_files $uri $uri/ /blog/index.php?$args;
        }

        location /forum {
                try_files $uri @vanilla;
        }

        location @vanilla {
                rewrite ^/forum(/.*) /forum/index.php?p=$1&$args last;
        }

        location ~ \.php$ {
                fastcgi_split_path_info ^(/)(/.*)$;
        }

        location ^~ /family {
                root   /app/mywebsitename/htdocs;
                index  index.html;
                auth_basic "Restricted Content";
                auth_basic_user_file /app/.htpasswd;
        }

        location ^~ /labs/ExperimentSite {
                root   /app/mywebsitename/htdocs;
                index  index.html;
                auth_basic "Restricted Content";
                auth_basic_user_file /app/.htpasswd;
                
                try_files $uri $uri/ /labs/ExperimentSite/index.html?$args;
        }

}

What have I missed here to get my /labs/ExperimentSite working? How can I get php working in html (I tried changing location ~ \.php$ to location ~ \.(php|htm|html)$ but that just broke everything!)

You have used the ^~ operator in location ^~ /labs/ExperimentSite to prevent any other location causing the "Restricted Content" to be bypassed.

Unfortunately, this also prevents the location ~ \.php$ block from handling any request that ends with .php and sending it to the PHP processor.

To handle .php files within /labs/ExperimentSite you could use a nested location.

For example:

location ^~ /labs/ExperimentSite {
    root   /app/mywebsitename/htdocs;
    index  index.html;
    auth_basic "Restricted Content";
    auth_basic_user_file /app/.htpasswd;
    
    try_files $uri $uri/ /labs/ExperimentSite/index.html?$args;

    location ~ \.php$ {
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass phpfpm-mywebsitename:9000;
        ...
    }
}

To send .html files to PHP, you should change the regular expression from \.php$ to \.(php|html)$.