PHP's source code is hosted on GitHub. To build from source you will first need to check out a working copy of the code.
mkdir /usr/local/src/php-7.0/ cd /usr/local/src/php-7.0/ git clone -b PHP-7.0 https://github.com/php/php-src .
If you want to add a feature, it's best to create your own branch.
git checkout -b my_private_branch
Finally, configure and build PHP
./buildconf ./configure make make test make install
If configuration fails due to missing dependencies, you will need to use your operating system's package management system to install them (e.g.
apt, etc.) or download and compile them from source.