Nginx + CodeIgniter Posting To Controller 500 Error - codeigniter

Question

I previously had this website on an Ubuntu/Apache host which has now been moved to a Debian/Nginx host. When I moved to Nginx it seems the form which used to work no longer works
How the form works is that it's hosted on a Wordpress website which is located at www.mycompany.com. It then posts the data to a controller which is hosted on the same server but using virtual hosts to site1.mycompany.com.
When I post the data, the form just hangs, looking at the console shows
POST https://site1.mycompany.com/request_form/submit 500 (Internal Server Error)
The CodeIgniter controller is called request_form and the submit function starts like this (to give you an idea)
public function submit()
{
$return = array('error' => false, 'msg' => '', 'error_data' => array());
$validation_rules = array();
//$this->form_validation->set_rules('serial_no', 'Outdoor Serial No', 'required');
$this->form_validation->set_rules('enquiry_type', 'Enquiry Type', 'required');
$this->form_validation->set_rules('txt_email', 'Email', 'required');
if(!empty($this->input->post('txt_email')))
{
$this->form_validation->set_rules('txt_email','email','valid_email|callback_email_exist');
}
$this->form_validation->set_rules('txt_phone', 'Phone', 'required');
$this->form_validation->set_rules('txt_address', 'Address', 'required|callback_check_address_for_state');
$this->form_validation->set_message('check_address_for_state','Address is not valid!');
My Nginx Config for site1.mycompany.com is
server {
server_name site1.mycompany.com;
root /var/www/html/site1.mycompany.com;
index index.php index.html index.htm;
autoindex on;
client_max_body_size 50M;
error_page 404 /404.html;
error_page 500 502 503 504 /50x.html;
location / {
try_files $uri $uri/ /index.php;
}
location = /50x.html {
root /usr/share/nginx/html;
}
location ~ \.php$ {
try_files $uri =404;
fastcgi_split_path_info ^(.+\.php)(/.+)$;
fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param SCRIPT_NAME $fastcgi_script_name;
}
}
There are no error logs in either /var/logs/nginx nor under CodeIgniter even when full logging is enabled. I have tried a variety of changes to the Nginx config with no avail.

Answer 1

Just try with these basic settings. (for easy debug just add echo "works"; die; in the home controller and check.)
In Ngnix
server {
server_name site1.mycompany.com;
root /var/www/html/site1.mycompany.com;
index index.html index.php;
# set expiration of assets to MAX for caching
location ~* \.(ico|css|js|gif|jpe?g|png)(\?[0-9]+)?$ {
expires max;
log_not_found off;
}
location / {
# Check if a file or directory index file exists, else route it to index.php.
try_files $uri $uri/ /index.php;
}
location ~* \.php$ {
fastcgi_pass 127.0.0.1:9000;
include fastcgi.conf;
}
}
In Config.php
$config['base_url'] = "http://site1.mycompany.com/";
$config['index_page'] = "";
$config['uri_protocol'] = "REQUEST_URI";