I don't think he wants to style the php code, but the content of his website, which is done by applying css to the output html.

PHP can be used to generate the html (or even the css), but the function highlight_string() has nothing to do with that.

Edit: BTW, if you even needed to use highlight_string(), I would recommend simply downloading Notepad++. It applies similar highlighting as you type - very useful for keeping track of your syntax