Note, selecting 'libapache2-mod-php7.3' for regex 'php7.3'

Perhaps the support wasn't available when you asked the question but all you 
need to do is as described in your Q.

Add ondrej PPA repository to your system.

1. sudo apt install software-properties-common
2. sudo add-apt-repository ppa:ondrej/php
3. sudo apt update

nstall php7.3 and check it's installed.
 1. sudo apt install -y php7.3
 2. php -v
 
This also works for php 7.4, php7.2 and php5.6. Just change the relevant digits.