PHP script won't execute bash script when called from webpage (NGINX web server)

I'm trying to setup a web interface for some controlled plug sockets I have, the sockets are controlled by a Raspberry Pi. I want to have a couple of links on a web page that I can use to turn the switches on and off. I've been trying to use PHP to do this and for some reason it just won't work.

I've tried various suggestions (see below links). All I'm getting is a white page whenever I click the link, and it doesn't do what its supposed to i.e. turn the switch on and off. Running the PHP script from the command line works as expected, the issue seems to be only when trying to run it from the webpage.

I've looked at the permissions and for the script I've set the permissions with: chmod 777 /path/to/script

I've tried storing the script in my home folder and in the /var/www/html folder with no joy. Nothing appears in the NGINX logs or PHP-FPM log to indicate any error.

I've tried editing the sudoers file to give www-data access to the script (www-data ALL:=/path/to/script/ and even tried it with all permissions for www-data (www-data ALL=(ALL:ALL) ALL) neither have worked.

I did think it might be because the script I'm trying to run involves starting an SSH session but I can't even get a local command to work to create a blank file either in the /home/pi/ directory or /var/www/html.

I've put the script I'm trying to run below along with the PHP I'm using to call the script and a second PHP file I've used to try other commands.

Any help or pointers in the right direction would be appreciated. I think the script is running but its failing somewhere and I can't work out where. The only thing I get back in a web browser is the echo $username line so I know its working in part but when I try to execute a command nothing happens.

PHP SCRIPT:

<?php
   $username = posix_getpwuid(posix_geteuid())['name'];
   echo $username;
   exec("/home/pi/scripts/switch2off");
?>

TEST SCRIPT:

<?php
  exec("touch /var/www/html/s/test.txt");
?>

SWITCH2OFF SCRIPT

#! /bin/bash
ssh pi@example 'python /home/pi/switches/switch_2_off.py'

NGINX CONFIG:

limit_req_zone $binary_remote_addr zone=one:10m rate=1r/s;

server {
        listen 80;
        listen [::]:80;
        return 301 https://$server_name$request_uri;

        root /var/www/html;

        index index.html index.htm index.nginx-debian.html;

        server_name example.com;

        location /.well-known/ {
                allow all;
        }
}

server {
        listen 443 ssl;
        listen [::]:443 ssl;

        server_name example.com;

        include snippets/ssl-example.conf;
        include snippets/ssl-params.conf;

        root /var/www/html;

        location / {
                limit_req zone=one burst=5;
                root /var/www/html;
                auth_basic "Please Log In";
                auth_basic_user_file /etc/nginx/.htpasswd;
                proxy_set_header X-Content-Type-Options: nosniff;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                add_header X-Frame-Options "allow-from example.com";
        }

        location /.well-known/ {
                allow all;
        }

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                include fastcgi_params;
        }

}

server {
    error_page 401 403 404 /404.html;
}

PHP-FPM LOG:

[03-Aug-2020 05:00:01] NOTICE: Terminating ...
[03-Aug-2020 05:00:01] NOTICE: exiting, bye-bye!
[03-Aug-2020 05:00:29] NOTICE: fpm is running, pid 620
[03-Aug-2020 05:00:29] NOTICE: ready to handle connections
[03-Aug-2020 05:00:29] NOTICE: systemd monitor interval set to 10000ms

MY RESEARCH/THINGS I'VE TRIED:

Nginx serves .php files as downloads, instead of executing them - I started here as initially I had a config issue when instead of running the PHP scripts it served them as a download instead.

Run a shell script with an html button - this is where I got the code from for the PHP script

PHP code is not being executed, instead code shows on the page - not quite the same issue as I'm seeing. The web browser doesn't display any code from the php file even when going to view source

https://askubuntu.com/questions/520566/why-wont-this-php-script-execute-bash-script

https://unix.stackexchange.com/questions/115054/php-shell-exec-permission-on-linux-ubuntu

https://www.linode.com/docs/web-servers/nginx/serve-php-php-fpm-and-nginx/