$ sudo add-apt-repository ppa:ondrej/php
$ sudo apt-get update
$ sudo apt-get upgrade php

$php  -v

PHP 7.1.15-1+ubuntu16.04.1+deb.sury.org+2 (cli) (built: Mar 6 2018 11:10:13) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
with Zend OPcache v7.1.15-1+ubuntu16.04.1+deb.sury.org+2, Copyright (c) 1999-2018, by Zend Technologies