Debian: Установка PHP 5.6

Тут есть инструкция по установке ключа

https://packages.sury.org/php/README.txt

apt-get update
apt-get -y install lsb-release ca-certificates curl
curl -sSLo /tmp/debsuryorg-archive-keyring.deb https://packages.sury.org/debsuryorg-archive-keyring.deb
dpkg -i /tmp/debsuryorg-archive-keyring.deb
sh -c 'echo "deb [signed-by=/usr/share/keyrings/deb.sury.org-php.gpg] https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list'
apt-get update
​apt install php5.6 php7.3