CentOS7 (and some of the other RHEL flavours) currently don’t include PHP 5.6+ in the core repos, and yet the versions of PHP bundled are at EOL or close to it.
A number of guides suggest using the webstatic or remi repos – but this is not recommended as they contain packages with names that conflict with packages in core repos.
One of the better options is to use the IUS repo (Inline with Upstream Stable project), which means you can quickly and easily update.
First, install the IUS rep (you will need the EPEL repo if you haven’t got it already)
yum install https://centos7.iuscommunity.org/ius-release.rpm
Next, we install, then use yum-plugin-replace to replace the existing PHP package
yum install yum-plugin-replace yum replace --replace-with php56u php
Finally httpd should be restarted, and you should be good to go
service httpd restart
DigitalOcean has a more detailed post on installing PHP7 from the same repo.