As of the 28 February 2014, Drupal 8 now requires a minimum PHP version of 5.4.2. For background information read the drupal.org issue.
This places everyone running Ubuntu 12.04 LTS in an awkward situation as the PHP version bundled with this release is PHP 5.3.10.
Luckily there are options to solve this:
Perform a dist-upgrade to 14.04 LTS
This may not be the easiest option, but I mention it for completeness, as this newer version of Ubuntu (Trusty Tahr) contains PHP 5.5.9 out of the box.
Add a PPA and install newer a newer version of PHP
For most people this will be the easiest option. For PHP 5.4.x run the command:
sudo add-apt-repository ppa:ondrej/php5-oldstable
or for PHP 5.5.x run:
sudo add-apt-repository ppa:ondrej/php5
And then update your packages:
sudo apt-get update sudo apt-get upgrade
The PPA maintainer has more information on the launchpad site https://launchpad.net/~ondrej/+archive/php5
I was getting the message "The following packages have been kept back" when running the upgrade command earlier
seanh /var/www/D8 git:8.x » sudo apt-get upgrade Reading package lists... Done Building dependency tree Reading state information... Done The following packages have been kept back: libapache2-mod-php5 linux-generic linux-headers-generic linux-image-generic php-pear php5-cli php5-common php5-curl php5-dev php5-gd php5-mcrypt php5-mysql php5-pgsql php5-xdebug
This was solved by manually installing the packages:
sudo apt-get install php-pear php5-cli php5-common php5-curl php5-dev php5-gd php5-mcrypt php5-mysql php5-pgsql php5-xdebug
Let me know if this worked for you in the comments, or if you have another way to easily update PHP on your stack.