Troubleshooting Nginx Issues After PHP Update: A Step-by-Step Guide

When you update your PHP version and Nginx stops working as expected, it usually indicates a configuration issue related to how Nginx communicates with PHP via PHP-FPM. Here are some troubleshooting steps to help resolve the issue:

1. Verify PHP-FPM Installation:

Ensure that the new PHP version installed includes PHP-FPM. You can check this by running the command:
```
php-fpmX.X -v
```
Replace X.X with your new PHP version number to ensure it’s installed correctly.

2. Update Nginx Configuration:

Update the Nginx configuration file to point to the correct PHP-FPM socket or TCP port of the new PHP version.
If you've specified a Unix socket, update it accordingly:
```
fastcgi_pass unix:/var/run/php/phpX.X-fpm.sock;
```
If you’re using TCP, ensure it reflects the correct port, often something like 127.0.0.1:9000.

3. Check PHP-FPM Service:

Ensure that the PHP-FPM service for the new version is running:
```
sudo service phpX.X-fpm status
```
If it's not running, start it:
```
sudo service phpX.X-fpm start
```

4. Look for Configuration Errors:

- Check the Nginx error logs for messages related to configuration errors, which can provide clues:
  ```bash
  sudo tail -f /var/log/nginx/error.log
  ```
- Similarly, check the PHP-FPM logs:
  ```bash
  sudo tail -f /var/log/phpX.X-fpm.log
  ```

5. Reload Nginx:

After making changes to Nginx or PHP-FPM configurations, always reload Nginx:
```
sudo systemctl reload nginx
```

6. Verify PHP-FPM Configuration Files:

Ensure that PHP-FPM configuration files (pool configuration located typically in /etc/php/X.X/fpm/pool.d/) are correctly set up with directories and user permissions that Nginx can access.

7. PHP-FPM Socket Permissions:

Verify that the socket file permissions and the directory containing it are set so that the Nginx user has access.

8. Firewall/SELinux Configuration (If Applicable):

Make sure that Nginx has the necessary permissions and that no security policies are blocking the communication between Nginx and PHP-FPM.

By systematically going through these steps, you should be able to identify and resolve the configuration issues that may have arisen during your PHP update. If problems persist, detailed error logs are often the best resource to diagnose further issues related to server configuration.