How can I use PHP 5.6 and Hestia?

I don’t know what you did, I suppose you installed php using apt but that is not the right way to do it, the right way is to use Hestia’s command v-add-web-php

v-add-web-php 5.6

If you want to use php 5.6 as the default php version:

v-change-sys-php 5.6

 sudo systemctl restart apache2

sudo systemctl restart apache2  # For Apache

sudo systemctl restart nginx    # For Nginx

systemctl restart php5.6-fpm

 /usr/local/vesta/bin