Nest is a nestable server templating library. It lets you create pages that look like standard html and easily add functionality with custom tag libraries. Nest templates are well-formed html that is then compiled into standard php. A Phing task has been included so that in a production environment all the templates can be pre-parsed and only the php files pushed to the production web server.
Nest is based on some of the concepts of JSP 2.0. Templates are well-formed html. Tag libraries are included by namespace. There are tags and tag files, and there is a simple expression language typing everything together.
While some of the better concepts of JSP have been carried over, Nest was designed first and foremost to be a PHP templating library.
Unlike JSP, Nest was designed to make it easy for web designers to add new tags. The level of programming skill needed with JSP to add a simple styled div tag was simply to high. It's easier now, but most web designers have moved on.
The PHP templating libraries I've looked at are often like writing in a different language. I tried using just PHP, but found it got too difficult to read even when using shrort tags. I moved to using xslt, but that didn't have a particularly good capacity for dealing with nested tags. So Nest was born.
Nest's tag libraries make it easy to encapsulate complex functionality and then simply work on the design concerns.