Upgrade php 7.3 Ubuntu 18.04 LTS

Since Ubuntu 18.04 comes with php 7.2, you must install from the ppa:ondrej repository: 

 sudo add-apt-repository ppa:ondrej/php

sudo apt update

sudo apt install php7.3 

 Install all the existing php 7.2 modules on php 7.3: 

 sudo apt install $(apt list --installed | grep php7.2- | cut -d'/' -f1 | sed -e 's/7.2/7.3/g') 

 Remove old PHP version: 

 apt purge php7.2 php7.2-common 

 If running Apache disable existing Apache php7.2 mod: 

 a2dismod php7.2 

 Enable Apache php7.3 mod: 

 a2enmod php7.3 

 Restart Apache: 

 systemctl restart apache2