By "highlight code" do you mean something along the lines of this:<?php
code goes in here...
?>
Or do you mean something more along the lines of a highlighter on a piece of paper, i.e. changing the background color of a span?
Also, are you looking to make this functionality automatic, or are you just looking for a style that you can use? In other words, are you looking for a plugin that modifies specific content upon document rendering, or are you content with just using span and or div tags?