You will have to download a script from ww.atomicorp.com and have to install it which will then allow you to upgrade PHP using yum.
1) Download the script:
wget -q -O – http://www.atomicorp.com/installers/atomic.sh
2) Install the script:
sh atomic.sh
3) Now upgrade PHP using yum:
yum upgrade php
Once it is installed, restart the httpd service.