First of all, we get the server key:

cd /tmp
cat dotdeb.gpg | apt-key add -

Then add the repository:

echo "deb wheezy all" >> /etc/apt/sources.list
echo "deb-src wheezy all" >> /etc/apt/sources.list

Change wheezy to the current debian release name

Then install php5-fpm:

apt-get update
apt-get install php5 php5-fpm php-pear php5-common php5-mcrypt php5-mysql php5-cli php5-gd php5-suhosin

More on php5-fpm: