In order to get php 5.6+ on RHEL7, we use the remi repo to supply this.
Download the rpm for remi that contains the repo information and keys:
# rpm -ivh remi-release-7.rpm
Now, edit the repo file and enable the e.g. php56 repo:
# vim /etc/yum.repos.d/remi.repo
Now you may install php from yum:
# yum install php php-mysql