Based on this table and chart I got from php.net, php version 5.6 will only be supported until the end of 2018.
If you decide still want to use php for your web application then I suggest you better get along with php version 7.x from now. Besides, the are many new features added to version 7.x, and it said it has better performance than version 5.x. Learn, and update your application from now to get compatible with version 7.x.
I’ll show you how to install php version 7.1 on Debian 8 as fpm service.
Add this repository to your system
Update your Debian 8 with this PPA packages by adding ppa: ondrej / php to the software sources of your system. Just remember, you need root permission to do this.
apt-get install apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list
Install PHP 7.1 fpm
apt-get install php7.1-fpm
Once done, you can check by type php -v
vagrant@debian:/tmp$ php -v PHP 7.1.11-1+0~20171027135825.10+jessie~1.gbp2e638d (cli)... Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies with Zend OPcache v7.1.11-1+0~20171027135825...
Congrats, now you have php 7.1 installed on your machine.