List all php packages installed:
dpkg -l | grep php| awk '{print $2}' |tr "\n" " "
Remove all php package:
sudo aptitude purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`
Add PPA
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update sudo apt-get install php5.6
You can install php5.6 modules too for example
sudo apt-get install php5.6-mbstring php5.6-mcrypt php5.6-mysql php5.6-xml
Verify your version
sudo php -v