Step 1
Add GPG keys for dotdeb.org:
wget http://www.dotdeb.org/dotdeb.gpg
cat dotdeb.gpg | apt-key add -
Step 2
Register dotdeb.org packages through /etc/apt/sources.list:
deb http://packages.dotdeb.org jessie all
deb-src http://packages.dotdeb.org jessie all
Step 3
Update the packages and upgrade the environment.
aptitude update
aptitude full-upgrade
Step 4
Install PHP 7 CLI and FPM:
aptitude install php7.0-cli php7.0-fpm
Step 5
Install the PHP extensions required for Magento 2:
aptitude install php7.0-bcmath php7.0-mysql php7.0-curl php7.0-gd php7.0-imagick php7.0-intl php7.0-mbstring php-soap php7.0-xmlrpc php7.0-xsl mcrypt php7.0-mcrypt php7.0-dev zip php7.0-soap php7.0-zip
pecl install timezonedb