Let’s have a look which packages are already installed and show only those with „php“ in the name. A fresh system usually shouldn’t give any results.
dpkg --get-selections | grep php
Next you can check what kind of modules are available. Do it like this:
sudo apt-cache search --names-only php7
Now install the php7 base packages with the command:
sudo apt-get install -y php7.0
Now install some other packages for php:
sudo apt-get install -y libapache2-mod-php7.0 php7.0-curl php7.0-xml php7.0-mysql php7.0-mcrypt php7.0-intl php7.0-zip php7.0-bz2