Centos 7 comes with PHP 5.4, securitywise this is not a problem, fixes get backported by RedHat, but Owncloud X (and likewise Nextcloud) need at least PHP 5.6. And naturally other web apps may also gain from newer PHP versions.
There is a more or less official way to have newer PHP versions on Centos and RHEL, Software Collections. There is PHP 5.6 and PHP 7.0. A slightly less official way for a current version is the Remi repository with PHP 7.1.
The Software Collection-way of installation is parallel installation, this means the newer PHP ist installed side by side to the system’s version (look in
/var/opt paths) and is only used if called explicitly. This is very clean but not very practical. You easily find howtos on globally changing the system’s PHP Version to the new one, especially for the Apache httpd Server, the main consumer of PHP. Remi also a has “replacing” way of installation.
But what if other web apps like Kolab won’t work with anything else than PHP 5.4?