Installing PHP 7.0 is easier than ever.  Here are instructions for installing the latest version on different platforms:

Ubuntu 14.04, 15.04, 15.10, and 16.04:

PHP 7.0 can be installed using Ondřej Surý's PPA:

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

View full list of available packages

Debian 8 (Jessie)

PHP 7.0 can be installed using the Dotdeb repository.

Add these two lines to your /etc/apt/sources.list file:

deb http://packages.dotdeb.org jessie all
deb-src http://packages.dotdeb.org jessie all

Add the GPG key:

wget https://www.dotdeb.org/dotdeb.gpg
sudo apt-key add dotdeb.gpg

Install PHP 7:

sudo apt-get update
sudo apt-get install php7.0

View full list of available packages

Debian 7 (Wheezy)

PHP 7 can be compiled from source by following the instructions outlined here: Install PHP 7 on Debian Wheezy

CentOS / RHEL

PHP 7.0 can be installed using the Webstatic Yum repository.

If you're using CentOS/RHEL 7.x, run these three commands to add the repository and install PHP 7:

rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
rpm -Uvh https://mirror.webtatic.com/yum/el7/webtatic-release.rpm
yum install php70w

If you're using CentOS/RHEL 6.x, run these two commands to add the repository and install PHP 7:

rpm -Uvh https://mirror.webtatic.com/yum/el6/latest.rpm
yum install php70w

View full list of available packages

Mac OS X 10.6 - 10.11

PHP 7.0 can be installed using homebrew:

brew tap homebrew/dupes
brew tap homebrew/versions
brew tap homebrew/homebrew-php
brew install php70

Or you can install it via Liip's php-osx tool:

curl -s http://php-osx.liip.ch/install.sh | bash -s 7.0

Windows

PHP 7.0 distributions for Windows can be found on the windows.php.net website: http://windows.php.net/download#php-7.0

You can find instructions for installing the distribution here: https://www.webtechgadgetry.com/install-php-7-windows/