apt-get install apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" >> /etc/apt/sources.list.d/php.list
apt-get update
apt-get install php5.6
update-alternatives --config php

root@nfsen:/# update-alternatives --config php
There are 2 choices for the alternative php (providing /usr/bin/php).

  Selection    Path             Priority   Status
------------------------------------------------------------
  0            /usr/bin/php7.3   73        auto mode
* 1            /usr/bin/php5.6   56        manual mode
  2            /usr/bin/php7.3   73        manual mode

Press <enter> to keep the current choice[*], or type selection number:

apachectl -M
a2dismod php7_module
a2enmod php5_module