Unlike Apache's decentralized .htaccess system, Nginx employs a centralized configuration structure. The primary files you'll work with are:

• /etc/nginx/nginx.conf - Main configuration file (handles global settings)
• /etc/nginx/sites-available/ - Contains virtual host configurations
• /etc/nginx/sites-enabled/ - Symbolic links to active configurations

For CodeIgniter and PHP applications, /etc/nginx/sites-available/default (or your custom site configuration) is indeed the correct file for implementing:

server {
    listen 80;
    server_name example.com;
    root /var/www/html;
    
    # CodeIgniter front controller pattern
    location / {
        try_files $uri $uri/ /index.php?$args;
    }

    # PHP handling
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    }
}

Three critical architectural differences:

1. Nginx requires server reload (sudo service nginx reload) for changes
2. No per-directory configuration files like .htaccess
3. Rewrite rules are written in server/location contexts

Here's a production-ready setup for Ubuntu:

server {
    listen 80;
    server_name yourdomain.com;
    root /var/www/ci_project/public;
    index index.php;
    
    # Security headers
    add_header X-Frame-Options "SAMEORIGIN";
    add_header X-XSS-Protection "1; mode=block";
    
    # Main rewrite rule
    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }
    
    # PHP processing
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    }
    
    # Block access to hidden files
    location ~ /\. {
        deny all;
    }
}

When facing 404 errors or routing problems:

1. Verify root points to your public directory
2. Check PHP-FPM socket path matches your PHP version
3. Ensure proper filesystem permissions (www-data:www-data)

Test your configuration with sudo nginx -t before reloading.

Unlike Apache which uses per-directory .htaccess files, Nginx employs a centralized configuration approach. The primary configuration files are typically found in these locations on Ubuntu systems:

/etc/nginx/nginx.conf (main configuration file)
/etc/nginx/sites-available/ (virtual host configurations)
/etc/nginx/sites-enabled/ (symlinks to enabled sites)
/etc/nginx/conf.d/ (additional configuration snippets)

For CodeIgniter and PHP applications, you'll primarily work with the virtual host file in /etc/nginx/sites-available/. The default file you mentioned (/etc/nginx/sites-available/default) is indeed the correct starting point.

Here's a typical structure for a PHP application:

server {
    listen 80;
    server_name example.com;
    root /var/www/html;
    index index.php;
    
    location / {
        try_files $uri $uri/ /index.php?$args;
    }
    
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    }
    
    location ~ /\.ht {
        deny all;
    }
}

For CodeIgniter applications, you need to handle the URI routing properly. Here's a complete working example:

server {
    listen 80;
    server_name ci-app.example.com;
    root /var/www/ci-app/public;
    
    index index.php index.html;
    
    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }
    
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_index index.php;
    }
    
    location ~ /\.ht {
        deny all;
    }
    
    location ~* \.(jpg|jpeg|gif|css|png|js|ico|html|xml|txt)$ {
        expires max;
        log_not_found off;
    }
}

• Nginx configurations are server-wide, not per-directory
• Changes require server reload (sudo systemctl reload nginx)
• Syntax is completely different from Apache's directives
• No .htaccess-style override capability by default

Problem: CodeIgniter routes not working properly

Solution: Ensure your try_files directive includes the query string:

try_files $uri $uri/ /index.php?$query_string;

Problem: PHP files being downloaded instead of executed

Solution: Verify your PHP-FPM socket path and include the proper fastcgi_params:

fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
include fastcgi_params;