Many developers encounter this confusing error when their Apache server can't communicate with PHP-FPM. The key symptom is that Apache looks for /usr/lib/cgi-bin/php5-fcgi but finds nothing there. Here's what's actually happening:

# Error example from logs:
[error] FastCGI: failed to connect to server "/usr/lib/cgi-bin/php5-fcgi"
[error] FastCGI: incomplete headers received

The confusion stems from how FastCGI was historically configured. The php5-fcgi file was never meant to physically exist - it's a virtual endpoint. The critical part is the socket connection defined in your php-fpm.conf:

FastCgiExternalServer /usr/lib/cgi-bin/php5-fcgi 
  -socket /var/run/php5-fpm.sock 
  -pass-header Authorization

For current systems (PHP 7.4+), a more reliable approach uses ProxyPassMatch:

    
        SetHandler "proxy:unix:/var/run/php/php7.4-fpm.sock|fcgi://localhost"
    

First check if PHP-FPM is running and listening:

sudo systemctl status php7.4-fpm
sudo ls -l /var/run/php/php7.4-fpm.sock

Then test socket permissions (www-data typically needs access):

sudo chown www-data:www-data /var/run/php/php7.4-fpm.sock
sudo chmod 660 /var/run/php/php7.4-fpm.sock

Here's a complete virtual host configuration that works with PHP 8.2:

    ServerName example.com
    DocumentRoot /var/www/html

    
        Options -Indexes +FollowSymLinks
        AllowOverride All
        Require all granted
    

    
        SetHandler "proxy:unix:/run/php/php8.2-fpm.sock|fcgi://localhost/"
    

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

1. Verify PHP-FPM pool configuration matches socket path
2. Check Apache modules are enabled: proxy_fcgi, proxy, rewrite
3. Confirm SELinux/apparmor isn't blocking socket access
4. Test with a simple phpinfo() script

When Apache complains about missing /usr/lib/cgi-bin/php5-fcgi while using PHP-FPM, it's actually a red herring. The real culprit lies in the FastCGI socket communication between Apache and PHP-FPM. Here's why the error occurs:

# The error suggests missing files, but the actual issue is socket communication
[error] FastCGI: failed to connect to server "/usr/lib/cgi-bin/php5-fcgi"
[error] FastCGI: incomplete headers received

Let's analyze the current setup in /etc/apache2/conf.d/php-fpm.conf:

<IfModule mod_fastcgi.c>
    AddHandler php5-fcgi .php
    Action php5-fcgi /php5-fcgi
    Alias /php5-fcgi /usr/lib/cgi-bin/php5-fcgi
    FastCgiExternalServer /usr/lib/cgi-bin/php5-fcgi -socket /var/run/php5-fpm.sock -pass-header Authorization
</IfModule>

For Ubuntu systems with PHP-FPM, we should use mod_proxy_fcgi instead of the deprecated mod_fastcgi:

# First, enable required modules
sudo a2enmod proxy_fcgi
sudo a2enmod actions

# Then update your configuration
<FilesMatch "\.php$">
    SetHandler "proxy:unix:/var/run/php/php7.4-fpm.sock|fcgi://localhost"
</FilesMatch>

After making changes, verify these critical points:

# Check if PHP-FPM is running
sudo systemctl status php7.4-fpm

# Verify socket file exists
ls -la /var/run/php/php7.4-fpm.sock

# Confirm Apache can access the socket
sudo -u www-data ls -la /var/run/php/php7.4-fpm.sock

If problems persist, check these common pitfalls:

# 1. Socket permissions
sudo chmod 666 /var/run/php/php7.4-fpm.sock

# 2. Configuration mismatch between PHP-FPM and Apache
# In /etc/php/7.4/fpm/pool.d/www.conf:
listen = /var/run/php/php7.4-fpm.sock
listen.owner = www-data
listen.group = www-data
listen.mode = 0660

If you must use mod_fastcgi, ensure proper socket configuration:

FastCgiExternalServer /usr/lib/cgi-bin/php5-fcgi -socket /var/run/php5-fpm.sock -idle-timeout 300 -pass-header Authorization

# Create the directory structure (even though it's not actually used)
sudo mkdir -p /usr/lib/cgi-bin/
sudo touch /usr/lib/cgi-bin/php5-fcgi
sudo chown www-data:www-data /usr/lib/cgi-bin/php5-fcgi