So, today I wanted to upgrade the PHP version installed in my notebook. 7.3 was not available at the default repositories, so I had to include a new one:

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

After which you'll refresh the cache

$ sudo apt update

Now you can install PHP7.3 normally

$ sudo apt install php7.3

And you can also install some extensions you know you'll need

sudo apt install php7.3-bcmath php7.3-bz2 php7.3-curl php7.3-gd php7.3-intl php7.3-json php7.3-mbstring php7.3-zip php7.3-fpm

My system was automatically reconfigured to use 7.3 instead of 7.2 and I can safely remove it

$ sudo apt remove php7.2

You can check the version in use with php -v