There are a couple of different ways to install php7 on centos7 – Installing yum-plugin-replace and inline-replaceing the whole php subsystem seems to be the easiest way:

You need to have a couple or yum repo’s enabled for this to work properly: webtactic and epel

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