php -v and phpinfo(); returns different versions

I want to use php 8.1 on Ubuntu server Currently it works with php 7.4.30

lsb_release -a

returns

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:    18.04
Codename:   bionic

I followed tutorials and so I executed these commands

sudo apt update
sudo add-apt-repository ppa:ondrej/php
sudo apt update
sudo apt install php8.1
sudo apt-get install -y php8.1-cli php8.1-common php8.1-mysql php8.1-zip php8.1-gd php8.1-mbstring php8.1-curl php8.1-xml php8.1-bcmath
sudo systemctl restart apache2

Now when I do php -v I see:

PHP 8.1.18 (cli) (built: Apr 14 2023 04:39:07) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.18, Copyright (c) Zend Technologies
    with Zend OPcache v8.1.18, Copyright (c), by Zend Technologies

when I do

sudo update-alternatives --config php

 Selection    Path             Priority   Status
---------------------------------------------
* 0            /usr/bin/php8.1   81        auto mode
  1            /usr/bin/php7.3   73        manual mode
  2            /usr/bin/php7.4   74        manual mode
  3            /usr/bin/php8.1   81        manual mode

Same for sudo update-alternatives --config phar and sudo update-alternatives --config phar.phar , php 8.1 is selected.

However, my application (a CMS) is still in php 7.4 I also tested with a simple php file with phpinfo();

What else can I check?

Probably you'll need to check php.ini.

In phpinfo() page, you will see which php.ini your web server is using. If you're seeing 7.4, probably your php.ini path would be something like:

.../7.4/apache2/php.ini

Just find the php.ini file for your php v8.1 installation and change the settings in your web server so it points to it.

PHP CLI is not the same as the PHP Apache module.

See this answer for more informations

So the solution is to tell Apache to use the installed version of PHP 8.1 by disabling the old PHP module and enabling the new PHP module using the following command.

sudo a2dismod php7.4
sudo a2enmod php8.1