Apache on Debian - Upgrading to PHP8.1 - phpinfo() not showing new version and conflicts with mpm

I think that your thrid question answers second, combined them into one list of commands

Why did my site go offline after I disabled the PHP8.1 module?

When you switch off apache php module, web server treats php code as plain text, which is a default behavior for unknown content type. Its kinda working, but your code is not interpreted at all.

And, how do I prevent that from happening again?

In order to prevent that either use varnish, load balancer(apache/nginx), make snapshots of the system when its stable or containerize your application with docker. Whatever will be suitable for your needs.

In varnish case it will create cached versions of your web pages and will return them in response omitting your webserver until no such cached version of web page exist(no cache hits in other words). But before, you need to write script which will cache all pages. Then varnish will simply send html responses.

In load balancer case if one node fail then it will redirect traffic to another alive node. Each node is basically a copy of your website.

Snapshots are images of your OS which you can use in different VMs for example. So if you messed up then you can simply delete snapshot and install initial one again.

In docker you can simply split your app into 2 services, which you can configure however you want without being afraid of making everything bad )

1 service will apache2

2 service will be php-fpm

Then you can simply build each service as an image and rotate images like in VM(Virtual Machine).

How do I upgrade to PHP 8.1-fpm?

# step 1, optional if you did it before
sudo apt update
sudo apt upgrade

# step 2, dont see any issues with simply installing newer version of php-fpm
sudo apt install php8.1-fpm

# check if user and group in php-fpm config are correct(/etc/php/8.1/fpm/pool.d/www.conf). 
# User and group should be the same as webserver user and group.
# if you want to check under which user webserver runs
# sudo ps aux| grep apache2 
listen.owner = www-data
listen.group = www-data
# and also check if socket/port for php-fpm are created/opened and its running normally

# step 3
# disable modules and restart configured services. For apache better to reload instead of restart
sudo a2dismod php8.1
sudo a2dismod mpm_prefork
sudo a2disconf php7.4-fpm
sudo a2enmod mpm_event
# optional in case its enabled
sudo a2enmod proxy
# optional in case its enabled
sudo a2enmod proxy_fcgi
sudo a2enconf php8.1-fpm
sudo systemctl restart php8.1-fpm
sudo systemctl restart apache2
# manually set php version for CLI
sudo update-alternatives --set php /usr/bin/php8.1
sudo update-alternatives --set phar /usr/bin/phar8.1
sudo update-alternatives --set phar.phar /usr/bin/phar.phar8.1

# step 4
# Mask the old PHP-FPM daemon (tell the system to not start the old php-fpm daemon)
sudo systemctl mask php7.4-fpm

Hope that something in this list will be helpful )

You need a package named php8.1-fpm to be able to use the mpm_event instead of mpm_prefork. For example this is mentioned in this tutorial:

Depending on the web server you use, you will need to install additional packages to integrate with the web server.

For Apache using mpm_event, ..., php8.0-fpm package provides integration with PHP 8.0 via FPM.

Of course you will want the php8.1-fpm package.

In my Apache config I had to change this line to use the right socket:

  <FilesMatch \.php$>
          SetHandler "proxy:unix:/var/run/php/php8.1-fpm.sock|fcgi://localhost/"
  </FilesMatch>

Infos about the socket:

 # ls -l /var/run/php/
total 4
-rw-r--r-- 1 root     root      5 Sep 22 12:42 php8.1-fpm.pid
srw-rw---- 1 www-data www-data  0 Sep 22 12:42 php8.1-fpm.sock=