在ubuntu16.04下无法安装php5问题解决

由于之前的项目是用php5写的，想要部署在升级到ubuntu16.04的系统上就要安装php5版本，运行下列命令后发现如下提示：

lhs@taing:/var/www/html$ sudo apt-get install php5
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package php5 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'php5' has no installation candidate

lhs@taing:/var/www/html$ sudo apt-get install libapache2-mod-php5

Reading package lists... Done
Building dependency tree... 50%
Building dependency tree       
Reading state information... Done
Package libapache2-mod-php5 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'libapache2-mod-php5' has no installation candidate

造成这种错误提示的原因是16.04官方软件库中没有php5，只自带php7。想要安装php5的话，需要安装14.04或者使用PPA，