PHP is the dominant programming language behind the majority of websites and platforms on the web (e.g. WordPress).
As such, it constantly undergoes updates and improvements.
It is important to keep your PHP version up-to-date to ensure that your website is as fast and secure as possible (as far as the PHP code is concerned, sites can have other vulnerabilities).
There are three main factors in determining which PHP version to use on your websites:
Here are the results of our own recent WPX Hosting speed tests on the different PHP versions (higher is better):

The speed difference between PHP 5.6 and PHP 7.0 is very significant – a 150% increase in requests per second.
The end result for you – if using PHP 7.X - is a much faster loading website.
Here are the results of our own recent WPX Hosting PHP memory usage tests (lower is better):

Though the improvement in memory usage is not as significant as the speed increase, it is still a very healthy 25%.
Let’s examine the support next.
The below chart shows the planned support timelines on the existing PHP versions and their statuses (courtesy of php.net):

As seen above, PHP 5.3, 5.4, 5.5 are already past the period for active support and critical security fixes. PHP 5.6’s active support finishes at the end of 2016.
This means that PHP 7.X will be the only ones fully supported for 2017 and onwards. WordPress also recommends updating your PHP to at least 7.0
https://wordpress.org/about/requirements/
With all this in mind, the decision to upgrade your sites to PHP 7.X should be an easy one.
Read here on how to upgrade if unsure (we can do it for you if you ask us to):
https://wpxhosting.com/knowledgebase/article/76/how-to-change-the-php-version-of-your-website/
Be sure to check that your websites are working under PHP 7.X If you encounter any problems, revert to your previous PHP version and let us know.
If at any point, you get stuck or aren't sure how to proceed with PHP changes for your sites, just raise a support ticket with us here: https://wpxhosting.com/tickets/new/ or talk to our 24/7 Live Chat team (widget in bottom right of your screen now) and we'll answer ASAP.