It is wrong and long way. Install standard PHP 5.6 package on system, and switch specified virtualhost to her. Here Ubuntu PPA: https://deb.sury.org/ , if not in our system.
Yes, that is also true, but it is not always possible immediately. But yes, if I am installing something new, then the most current version of that program. And if the most current version is the one that depends on php 5.6, I would watch out for that program and consider installing it.
Thanks, I will need to use this CMS so I will use another hosting provider to run 5.6 for the time being.
If all goes well I will put it on its own VPS