If you're on ubuntu 18.04 you may have noticed that its not available. Here are some quick steps to remedy that.
First make sure you've updated and upgraded your system.
sudo apt update sudo apt upgrade
Then we'll add this PPA to our repository:
sudo apt install software-properties-common sudo add-apt-repository ppa:ondrej/php sudo apt update
After the last update completes, you can check to see if php7.3 is available by doing an apt search:
sudo apt search php7.3
If you see 7.3 on versions of php and php modules as below:
you're ready to install php7.3 with the following command
php sudo apt install php7.3