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.
References:
https://www.dotdeb.org/instructions/

STEPS:
Edit the file /etc/apt/sources.lst and add the following lines:
deb http://packages.dotdeb.org wheezy all
deb-src http://packages.dotdeb.org wheezy all
deb http://packages.dotdeb.org wheezy-php56-zts all
deb-src http://packages.dotdeb.org 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 pgpkeys.mit.edu --recv-key E9C74FEEA2098A6E
gpg -a --export E9C74FEEA2098A6E | apt-key add -
apt-get update
apt-get upgrade
apt-get install php5