在Ubuntu16.04桌面系统安装php5时，提示Package php5 have no installation candidate，经过搜索，找到的解决该问题的答案：
Ubuntu 16.04 comes with PHP7 as the standard, so there are no PHP5 packages
However if you like you can add a PPA to get those packages anyways:
Add the PPA
sudo add-apt-repository ppa:ondrej/php
Install your PHP Version
sudo apt-get update
sudo apt-get install php5.6
You can install php5.6 modules too ..