I was looking for info on this myself. I read around, and found this set of upgrade instructions:
To use the IUS packages in CentOS 7:
1. Install EPEL and IUS repositories:
# yum --enablerepo=extras install epel-release
# yum install https://centos7.iuscommunity.org/ius-release.rpm
2. Install yum-plugin-replace and replace the php packages with php56u packages:
# yum install yum-plugin-replace
# yum replace --replace-with php56u php
3. Restart any services which depend on php, such as httpd.
Note that this will install the current version of 5.6, which at the time of posting is 5.6.16, and that a "yum update" will keep the packages up to date in future.
I'm not a complete server neophyte, but this is something I've not done before (use a different repository), so I asked Linode support about it. They had this to say:
It's hard to say if upgrading will break anything within your Linode as an infrastructure provider we do have our limitations as to what we can Support. But those steps you've listed look correct based my readings.
A subsequent question to them about "What's the official policy then" garnered this in reply:
The official stance by CentOS's maintainers is to update to the latest version of CentOS 7 and then update PHP accordingly. However, if you want to keep the same version of CentOS then using a different repository for PHP updates should be fairly safe. If you need to, you can always uninstall PHP and install it from a different repository or package.
I've personally used Remi's repository for Enterprise Linux, which covers RHEL and CentOS:https://rpms.remirepo.net/enterprise/7/
But I hear that IUS is also very reliable.
So at this point, I'm a bit confused as to what path to take.