The error "FastCGI sent in stderr: Primary script unknown" typically occurs when Nginx cannot properly communicate with PHP-FPM to execute the requested script. This is fundamentally a path resolution problem between the web server and PHP processor.

Let's analyze the critical components in the configuration that need attention:

location ~ \\.php {
    fastcgi_split_path_info  ^(.+\\.php)(.*)$;
    
    # Path resolution logic
    set $fsn /$yii_bootstrap;
    if (-f $document_root$fastcgi_script_name){
        set $fsn $fastcgi_script_name;
    }

    fastcgi_pass   127.0.0.1:9000;
    include fastcgi_params;
    fastcgi_param  SCRIPT_FILENAME  $document_root$fsn;
}

Here are the most frequent causes and their solutions:

1. Incorrect Document Root

Verify both Nginx and PHP-FPM are using the same document root:

# In nginx.conf
root /var/www/project/public;

# In php-fpm pool config (www.conf)
chdir = /var/www/project/public

2. Permission Issues

Instead of using 777 permissions, implement proper ownership:

sudo chown -R www-data:www-data /var/www/project
sudo find /var/www/project -type d -exec chmod 755 {} \;
sudo find /var/www/project -type f -exec chmod 644 {} \;

3. PHP-FPM Configuration

Ensure your pool configuration matches:

[www]
user = www-data
group = www-data
listen = 127.0.0.1:9000
listen.owner = www-data
listen.group = www-data
pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3

Here's a proven configuration that resolves the issue:

server {
    listen 80;
    server_name project.local;
    root /var/www/project/public;
    
    index index.php index.html;
    
    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }
    
    location ~ \\.php$ {
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\\.php)(/.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

Follow this diagnostic sequence:

1. Verify file exists: ls -la /path/to/script.php
2. Check Nginx error logs: tail -f /var/log/nginx/error.log
3. Confirm PHP-FPM is running: sudo systemctl status php-fpm
4. Test PHP-FPM directly: SCRIPT_FILENAME=/path/to/script.php REQUEST_METHOD=GET cgi-fcgi -bind -connect 127.0.0.1:9000

For complex setups:

• When using Docker, ensure volume paths match between containers
• For symlinked deployments, set realpath_root in Nginx
• With SELinux enabled, check context: ls -Z /path/to/script.php

---

When migrating from Apache to Nginx, the "Primary script unknown" error often catches developers off guard. The error message in your logs indicates Nginx successfully passed the request to PHP-FPM, but the FastCGI processor couldn't locate the actual script file.

From your configuration, I notice several potential issues:

# Problematic original configuration
fastcgi_param  SCRIPT_FILENAME  $document_root$fsn;
# While $document_root points to /var/www/, $fsn may contain incorrect path

First, verify these critical elements:

# Correct PHP location block example
location ~ \.php$ {
    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    fastcgi_pass 127.0.0.1:9000;
    fastcgi_index index.php;
    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}

Even with 777 permissions, SELinux might block access. Check:

ls -laZ /var/www/
ps aux | grep php-fpm
getenforce

Here's a proven configuration that works for PHP applications:

server {
    listen 80;
    server_name console.ordercloud;
    root /var/www/console/frontend/www;
    
    index index.php index.html;
    
    location / {
        try_files $uri $uri/ /index.php?$args;
    }
    
    location ~ \.php$ {
        include fastcgi_params;
        fastcgi_intercept_errors on;
        fastcgi_pass unix:/run/php/php-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

When troubleshooting:

# Check PHP-FPM pool configuration
grep -R "listen" /etc/php/*/fpm/pool.d/
# Verify Nginx and PHP-FPM user permissions
ps aux | grep -E 'nginx|php-fpm'

Developers often miss:

• Mismatch between fastcgi_pass address and PHP-FPM listen address
• Incorrect SCRIPT_FILENAME construction
• Missing try_files directive in PHP location block
• Path discrepancies between root directive and actual file location

Create a test PHP file:

echo "" > /var/www/console/frontend/www/test.php

Then access it via browser to confirm PHP processing works before debugging your application.