Install libsodium in PHP 7

libsodium.md

Ubuntu 16, from packages:

sudo apt-get install php7.1-sodium
sudo service apache2 restart

Alternative (https://lukasmestan.com/install-libsodium-extension-in-php7/)

sudo pecl install -f libsodium
sudo echo "extension = sodium.so" > /etc/php/7.1/mods-available/sodium.ini
sudo phpenmod sodium