php 7.4 specific install ubuntu 18.04 code example
Example 1: install php 5.6 on ubuntu 18.04
$ sudo add-apt-repository ppa:ondrej/php
Example 2: install php 7.1 on ubuntu 18.04
sudo apt install php7.1 php7.1-common php7.1-opcache php7.1-mcrypt php7.1-cli php7.1-gd php7.1-curl php7.1-mysql