如果需要更新源：
#vim /etc/apt/sources.list
在末尾加入
# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted deb-src http://archive.ubuntu.com/ubuntu xenial main restricted #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial universe deb http://mirrors.aliyun.com/ubuntu/ xenial-updates universe deb http://mirrors.aliyun.com/ubuntu/ xenial multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-updates multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse #Added by software-properties deb http://archive.canonical.com/ubuntu xenial partner deb-src http://archive.canonical.com/ubuntu xenial partner deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial-security universe deb http://mirrors.aliyun.com/ubuntu/ xenial-security multiverse
#apt-get update
#apt-get upgrade
#apt-get install php5
如果需要安装依赖
# apt-get install php5-fpm php5-gli php5-cgi libapache2-mod-php5filter libapache2-mod-php5
#apt-get install php5
停用原来版本
#a2dismod php7
启用刚安装的版本
#a2enmod php5
#service apache2 restart
最后用phpinfo()检查
试试这个：
sudo add-apt-repository ppa:ondrej/php
sudo apt-get install apache2
sudo apt-get install mysql-server mysql-client
sudo apt-get update
sudo apt-get install php5.6
sudo apt-get install libapache2-mod-php5.6
sudo apt-get install php5.6-mysql php5.6-mbstring php5.6-mcrypt php5.6-xml
sudo apt-get install php5.6-gd
sudo apt-get install php5.6-curl