This Guide explains how to change the php version for ALL folders / sites on your hosting account, if you want to change a specific site to a different version of php please see this article :

We have php 5.3 as the default version of php on our shared hosting servers

To use a different version of PHP (php 5.3  php 5.4 or php 5.5 ! ) simply log into your control panel, then in here click on 'Select PHP Version' , then in the drop down selector choose the version of PHP you would like to use, then click on 'Set as current'.

You can also then change the php extensions that are loaded

shows a table of possible php extensions that can be enabled / disabled, we recommend always ensuring your have the mysql / mysqli / pdo_mysql boxes ticked.

Once you have choosen the php extensions / modules you would like, then click the 'save' button at the bottom of the page to set those modules!

Then your done, enjoy the new version of php !