(Update)
Try this (Ubuntu):
sudo add-apt-repository ppa:ondrej/php -y
sudo apt-get update
sudo apt-get install php5.6-fpm -y
(Update)
For Debian Wheezy (with sudo)
echo "deb http://packages.dotdeb.org wheezy-php56 all" >> /etc/apt/sources.list.d/dotdeb.list
echo "deb-src http://packages.dotdeb.org wheezy-php56 all" >> /etc/apt/sources.list.d/dotdeb.list
wget http://www.dotdeb.org/dotdeb.gpg -O- | apt-key add -
apt-get update
apt-get install php5-cli php5-fpm ….. (or whatever package you might need)
(Update 21/06/2017)
For Debian 8 (jessie)
sudo nano /etc/apt/sources.list
Add the following repositories:
...
deb http://mirrors.digitalocean.com/debian jessie main contrib non-free
deb-src http://mirrors.digitalocean.com/debian jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb-src http://security.debian.org/ jessie/updates main contrib non-free
# jessie-updates, previously known as 'volatile'
deb http://mirrors.digitalocean.com/debian jessie-updates main contrib non-free
deb-src http://mirrors.digitalocean.com/debian jessie-updates main contrib non-free
Then update your sources:
sudo apt-get update
Then install the php5-fpm
sudo apt-get install php5-fpm