I am not that far into PHP, but one thing you could do is add ?skin=hi or =lo to the url depending on which version you are linking to. Then in the page that you have that added to (next.php?skin=hi for example) have something like
include ($skin/header.php);
Then, you would need a header.php in the hi/ directory and one in the lo/ directory. There are better ways to do this probably and other stuff you can do...

The main reason to have a high and low version is just in image heavy areas. Usually the header is the only one, but stuff like a big background image or title images could be done with an IF $skin == "hi" sorta thing then have it print either the image tag or just text. Trying to have a skin for either images for large sections of text or just regular text would be ridiulous. Like, if you wanted to have a version where all the text was an image, and one where it was text, that would be crazy. http://www.webmonkey.com has good stuff on php, http://www.php.net has a manual that is pretty helpful, and http://www.bloobseven.com has some php tuts that helped me get into mySql. Also, you probably know (but some people don't) that PHP works with html or other mark up languages. The php is just interpreted on the server and an html page is generated for the viewer to download. This means both that you have to use html (or the like) the same you would designing a regular page and that the viewer won't see the php code. My friend thought that he had to use php instead of html to design the site, and so he didn't want to bother with it. I don't even know where I am going with this anymore...