In order to upgrade PHP from 5.4 to 5.6 we need to use the DOTDEB repositories by which many other packages will also be upgraded. Here are the steps.

Edit the file /etc/apt/sources.lst and add the following lines:
deb wheezy all
deb-src wheezy all
deb wheezy-php56-zts all
deb-src wheezy-php56-zts all

Then run the following commands:
Note: Because of a bug in these package’s after-install I added some extra commands that will take care of it:
mkdir -p /etc/apache2/conf-available/
touch /etc/apache2/conf-available/php5-cgi.conf
gpg --keyserver --recv-key E9C74FEEA2098A6E
gpg -a --export E9C74FEEA2098A6E | apt-key add -
apt-get update
apt-get upgrade
apt-get install php5