Problem with Drupal 8.0 installation – PHP extensions disabled

Ubuntu 16.04 comes with PHP7 as the standard, so there are no PHP5 packages

However if you like you can add a PPA to get those packages anyways:

Remove all the stock php packages

List installed php packages with dpkg -l | grep php| awk '{print $2}' |tr "\n" " " then remove unneeded packages with sudo aptitude purge your_packages_here or if you want to directly remove them all use :

sudo aptitude purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`

Add the PPA

sudo add-apt-repository ppa:ondrej/php

Install your PHP Version

sudo apt-get update
sudo apt-get install php5.6

You can install php5.6 modules.

PHP7 packages for Ubuntu including php7.0-gd can be installed via PPA for PHP by Ondřej Surý:

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php7.0-gd

EDIT:

As MacroMan stated, under Ubuntu 16.04 you don’t need to add the repository any more. Simply issuing sudo apt-get install php7.0-gd works.

Install mod_proxy to get ProxyPass to work

In the end I guessed that the module proxy_http in the mods-enabled directory might work, and used the command:

a2enmod proxy_http

Verify your version

sudo php -v

Based on http://askubuntu.com/a/756186/532957 (thanks @AhmedJerbi)