In a nutshell, Less CSS is a tool that lets you simplify your CSS style sheet and allows you to extend limited CSS functionality by introducing variables, mixins, operations and nested rules. The original implementation is written in Ruby; however, there is a PHP solution based on LESS CSS called lessphp.
He mentions the good and the bad about using LESS - it can speed up your CSS development time by making it more familiar but it also comes with the caveat that the CSS files have to be generated each time an update is made. He covers some of the basic features of lessphp including variables, mixins, nested rules, operations (improving on already defined rules) and being able to use multiple .less files. He includes a XML snippet that can be dropped into your Ant (or Phing) build to run the generation at deployment time.