As of March 23 the default php version will become 5.6 progressively for each website.
Who created a blog or uses an application installed from AlterVista control panel is already using php 5.6, and will not notice any change. Who installed a custom application can in any moment change the php version in AlterVista control panel (Resources -> PHP Settings).
You are encouraged to update your applications in order to make them compatible with php 5.6 since older versions will be dismissed in the future.