I have a legacy application which was originally written in straight PHP with a few classes that I have created for convenience, but this is old and I am wanting to get this into a modern web framework. [...] I wanted to update my site with better validations anyway so I will start with flash messages and then validations. For flash messages in lithium you will need the li3_flash_message plugin.

He works through the different parts of the framework setup he needed including a simple bootstrap file and configuring it to use the li_flash_message library. He shows how to use validation rules on submitted data to ensure things like email an link format validity as well as spit back a message for any that are incorrect.