You can do this with update-alternatives. If you would like to do this interactively, you can do this:

sudo update-alternatives --config php

If you like to specifically choose the PHP version (via an alias or whatnot), you can do this:

sudo update-alternatives --set php /usr/bin/php7.4

Of course, be sure to change php7.4 to the actual version you want to switch between.

Interactive switching mode

sudo update-alternatives --config php
sudo update-alternatives --config phar
sudo update-alternatives --config phar.phar

Manual Switching

From PHP 5.6 => PHP 7.1

Default PHP 5.6 is set on your system and you need to switch to PHP 7.1.

Apache:

$ sudo a2dismod php5.6
$ sudo a2enmod php7.1
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php7.1
$ sudo update-alternatives --set phar /usr/bin/phar7.1
$ sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.1

From PHP 7.1 => PHP 5.6

Default PHP 7.1 is set on your system and you need to switch to PHP 5.6.

Apache:

$ sudo a2dismod php7.1
$ sudo a2enmod php5.6
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php5.6

Source

apt-get update updates only the package lists. To actually upgrade the packages you have to run:

sudo apt-get upgrade

or

sudo apt-get dist-upgrade

More info here.

If you ran the above commands but the packages still were not upgraded, probably you didn't add Ondřej Surý's PHP PPA to your system.

Run the following to add the PPA:

• For PHP 5.5, PHP 5.6 and PHP 7.0:

  sudo apt-add-repository ppa:ondrej/php
  

• For PHP 5.4 (Deprecated, upgrade at least to PHP 5.5):

  sudo apt-add-repository ppa:ondrej/php5-oldstable
  

• For PHP 5.5 (Deprecated, use ppa:ondrej/php):

  sudo apt-add-repository ppa:ondrej/php5
  

• For PHP 5.6 (Deprecated, use ppa:ondrej/php):

  sudo apt-add-repository ppa:ondrej/php5-5.6
  

Then update the package lists and perform the upgrade:

sudo apt-get update

For PHP 5.5:

sudo apt-get install php5.5

For PHP 5.6:

sudo apt-get install php5.6

For PHP 7:

sudo apt-get install php7.0

Release upgrade

If you want to install the latest PHP (for example PHP7) but you want to stick to the release's packages intead of hacking the sources around; and you want to do a full-release upgrade (for example you are in Ubuntu 14.04 -which does not have php7- and you want to go to Ubuntu 16.04 -which does- then you can do:

do-release-upgrade

Now you should have the latest PHP version.

It's hard getting the exact PHP 8.1.22 version. However, if you are OK with something close to it, 8.1.24 is the current version at ppa:ondrej/php.

• Remove your current PHP 8.1:

  sudo apt remove php8.1
  

• Install ondrej's repo:

  sudo add-apt-repository ppa:ondrej/php
  sudo apt update
  

• Install PHP 8.1:

  sudo apt install php8.1
  

• Disable the default PHP 8.2 in Ubuntu 22.04:

  sudo a2dismod php8.2
  

• Enable PHP 8.1:

  sudo a2enmod php8.1
  

• Restart the Apache:

  sudo service apache2 restart
  

Below is a description of what I did. I hope this information can help someone else:

I installed PHP 7.1 along side PHP 7.2. I also installed most of the needed extensions for PHP 7.1. I then did a2dismod php7.2 and a2enmod php7.1 so that I could switch over to PHP 7.1 while keeping 7.2 still installed on the server. Most of my sites work after making the switch. The only site that doesn't seem to be working is a Joomla site. The full list of commands I ran are below:

sudo add-apt-repository ppa:ondrej/php

sudo apt-get update

sudo apt-get install php7.1

sudo apt-get install php7.1-cli php7.1-common php7.1-json php7.1-opcache php7.1-mysql php7.1-mbstring php7.1-mcrypt php7.1-zip php7.1-fpm

sudo a2dismod php7.2

sudo a2enmod php7.1

sudo service apache2 restart