add the fallowing lines to  /etc/apt/sources.list

deb http://packages.dotdeb.org wheezy-php56 all
deb-src http://packages.dotdeb.org wheezy-php56 all

then

wget http://www.dotdeb.org/dotdeb.gpg
wget http://www.dotdeb.org/dotdeb.gpg
sudo apt-get update
sudo apt-get install php5

or if you have it already installed:

apt-get dist-upgrade
sudo service apache2 restart