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# reference1:  https://by-example.org/ubuntu-16-04-xenial-downgrade-php-7-to-php-5-6/
# reference2: https://askubuntu.com/questions/761713/how-can-i-downgrade-from-php-7-to-php-5-6-on-ubuntu-16-04/761735 
# reference2 (recommended)

sudo add-apt-repository ppa:ondrej/php

sudo apt-get update

sudo apt-get install php7.0 php5.6

# from php5 to php7
#sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart

# from php7 to php5 
#sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart

sudo update-alternatives --config php