# php -v
PHP 5.6.33 (cli) (built: Jan 14 2018 08:42:34)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
The above shows you the version of the command line php interpreter. It may happen that the version of the php module loaded by your web server is not the same.
When you want to know the version of php used by your web server, create a small file my_php_info.php and put that somewhere in your /var/www/html directory, so you can load it as a web page in your web browser. Give it the following contents:
Remove the file when you are done; it shows important configuration information that never should be shown on a public web site.
You write your VPS has 256 MB memory. That is rather low for a public website. You have to carefully go through your php.ini and mysql configuration files and adjust the limits for the amount of memory that php and mysql are allowed to use.
As TrevorH already mentioned: stay away from the webtatic repository. See https://wiki.centos.org/AdditionalResources/Repositories
for a list of good repositories.