LC_ALL=C.UTF-8 add-apt-repository ppa:ondrej/php
apt update
apt install php7.3 php7.3-cli php7.3-common

php -v
PHP 7.3.20-1+ubuntu16.04.1+deb.sury.org+1 (cli) (built: Jul 9 2020 16:33:48) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.20, Copyright (c) 1998-2018 Zend Technologies
with Zend OPcache v7.3.20-1+ubuntu16.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies