How to install php5.6 in the Debian 9
The following packages have unmet dependencies: libapache2-mod-php5 : Depends: libdb5.1 but it is not installable Depends: libonig2 (>= 5.2.0) but it is not installable Depends: libssl1.0.0 (>= 1.0.1) but it is not installable Depends: apache2-mpm-prefork but it is not installable or apache2-mpm-itk but it is not installable Depends: php5-cli but it is not going to be installed E: Unable to correct problems, you have held broken packages.
You can use the packages released by Ondřej Surý , see deb.sury.org
Open the terminal and run the following command:
apt-get install apt-transport-https lsb-release ca-certificates
Get the gpg key:
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
or:
curl https://packages.sury.org/php/apt.gpg | apt-key add -
Add the new repository to your sources:
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list
Install PHP5.6
apt-get update
apt-get install php5.6
To switch between PHP versions:
update-alternatives --config php
Sample output:
----------------------------------------------------------
* 0 /usr/bin/php7.0 70 mode automatique
1 /usr/bin/php5.6 56 mode manuel
2 /usr/bin/php7.0 70 mode manuel
来源：https://stackoverflow.com/questions/46378017/install-php5-6-in-debian-9