php7 was released in December last year, and ubuntu15.10 was released in October last year. In order to ensure that the software running on the system can run stably, there will be no major changes in the software version after the system is released. At present, php7 has been introduced into ubuntu16.04 and is expected to be released in April. Other distributions have similar reasons.

If you want to install php7 on the current version, it is recommended to use the ppa extension library

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php7.0

centos can install php7 through the ius extension library. For specific usage, please see https://ius.io/GettingStarted/