1、添加PPA源

sudo su -
echo "deb http://ppa.launchpad.net/ondrej/php/ubuntu xenial main" | tee -a /etc/apt/sources.list
echo "deb-src http://ppa.launchpad.net/ondrej/php/ubuntu xenial main" | tee -a /etc/apt/sources.list
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E5267A6C
apt-get update
exit

2、安裝PHP5.6
sudo apt-get install php7.2