Virtualmin PHP7 + PHP5 the right way

So I’ve been searching on the Internet for a while and made some tests but I don’t think it works the way I would expect.

By deafult Vmin comes with php7.0 but I would like to have two (or three) other versions - php5 and php7.2 for testing purposes. Well php5 is the one that I would need the most on my Debian Stretch

Can someone correct me if installing php5 requires only to add this repo:

Or there’s anything else that I need to do?
Why I’m asking? Cause when I want to install php-intl I get this:

apt-get install php-intl
Reading package lists… Done
Building dependency tree
Reading state information… Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
php-intl : Depends: php7.0-intl but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

What’s wrong and how to fix this?

So here’s an update.

Again on a fresh Stretch_x64 installation of Vmin I did the following:

$ apt-get install apt-transport-https
$ curl | apt-key add -
$ echo ‘deb stretch main’ > /etc/apt/sources.list.d/
$ apt-get update && apt-get upgrade -y

Next I installed php5.6:

$ apt-get install php5.6-cli php5.6-fpm

And since then the *.php handlers stopped working. PHP scripts are executed only in mod_php no matter which version of PHP I choose (5.6, 7.0, 7.1 or 7.2)

My big question is… How do I achieve backward-compatibility of PHP on my Vmin instance? I have two mini websites that still use php5 and they simply can’t work on php7 yet - they wait for total remake this year.