sudo add-apt-repository ppa:ondrej/php
sudo apt-get update

For PHP 5.5.36

sudo apt-get install php5.5

For PHP 5.6.22

sudo apt-get install php5.6

To verify the version installed, run the following command:

sudo php -v

If you want to remove PHP installed, then run the following commands:

sudo apt-get update
sudo apt-get purge php5-common -y
sudo apt-get --purge autoremove -y