apt-get install php7.0 php7.0-curl php7.0-gd php7.0-cli mysql-server php7.0-mysql php-xml php7.0-mbstring as doc said
apt-get install php7.0 php7.0-curl php7.0-gd php7.0-cli mysql-server php7.0-mysql php-xml php7.0-mbstring
https://piwik.org/docs/requirements/