I wasn't planning to upgrade PHP today, but in order to use Symfony 4.0, I had to.

First, I wanted to get all the PHP RPMs

sudo yum list installed php55u* | grep php | cut -f1 -d' ' | tr -d '.i686' | tr "\n" ' ' | sed "s/55/71/g" > php

Next, I removed all the PHP 5.5 RPMs:

sudo yum remove php55*

Then I edited the output file (php) and added a sudo yum install at the beginning of the file

So I could use

source php

There were a few more RPMs I needed, after roaming about the web for a bit, these were the commands that I ran

sudo yum install pear1u
sudo yum install php71u-json
sudo yum install libmemcached-devel
sudo pecl install memcached

The recklessly part of this is that I confess I did not check ... anything. My existing Symfony 3.3.10 application comes up and lets me log in. I haven't checked more than that.

Good luck!