Error “No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed” (SOLVED)

Debian and derivative distributions (Ubuntu, Linux Mint, Kali Linux, and many others) may experience a “FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed” error when migrating from PHP 8.1 to PHP 8.2.

Apache web server log

sudo tail /var/log/apache2/error.log

contains the following error messages:

[Sun Jan 29 03:05:45.213609 2023] [proxy:error] [pid 1313500] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed
[Sun Jan 29 03:05:45.213763 2023] [proxy_fcgi:error] [pid 1313500] [client 127.0.0.1:58950] AH01079: failed to make connection to backend: httpd-UDS
[Sun Jan 29 03:06:39.789031 2023] [proxy:error] [pid 1313496] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed
[Sun Jan 29 03:06:39.789110 2023] [proxy_fcgi:error] [pid 1313496] [client 127.0.0.1:50024] AH01079: failed to make connection to backend: httpd-UDS
[Sun Jan 29 03:06:41.336218 2023] [proxy:error] [pid 1313499] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed
[Sun Jan 29 03:06:41.336297 2023] [proxy_fcgi:error] [pid 1313499] [client 127.0.0.1:50040] AH01079: failed to make connection to backend: httpd-UDS
[Sun Jan 29 03:07:24.027400 2023] [proxy:error] [pid 1313497] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed
[Sun Jan 29 03:07:24.027445 2023] [proxy_fcgi:error] [pid 1313497] [client 127.0.0.1:44688] AH01079: failed to make connection to backend: httpd-UDS
[Sun Jan 29 03:09:55.008467 2023] [proxy:error] [pid 1504919] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php8.1-fpm.sock (*:80) failed
[Sun Jan 29 03:09:55.008530 2023] [proxy_fcgi:error] [pid 1504919] [client 127.0.0.1:38382] AH01079: failed to make connection to backend: httpd-UDS

Sites and engines that use PHP, such as phpMyAdmin, give the following error:

503 Service Unavailable
Service Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Apache/2.4.55 (Debian) Server at localhost Port 80

The essence of the problem is indicated in the web server logs – it is impossible to connect to the php8.1-fpm.sock socket. This is a fairly predictable problem when changing the PHP version, in this case from PHP 8.1 to PHP 8.2.

FPM (FastCGI Process Manager, FastCGI process manager) is an alternative implementation of PHP FastCGI with several additional features commonly used for high load sites.

To disable an outdated version of FPM, run the following commands:

sudo a2disconf php8.1-fpm
sudo systemctl restart apache2

At this point, engines and sites that don't use FPM should already be working fine.

If you are really using FPM and want to enable the new version, then run the following commands:

sudo a2enconf php8.2-fpm
sudo systemctl reload apache2
sudo systemctl restart apache2.service
sudo systemctl restart php8.2-fpm