About Skins and HTML Templates (as that is what you seem to propose) I had a few of my webpages running using Fast Template http://www.google.com/search?q=fast+template (a few tutorial + modifications can be seen on google) I don't really like it anymore, but it may be worth a look. The way you handle it is, making an external template file which is basically html-code with {SPECIAL_TAGS} in curly brackets which you can replace from your code. But for REAL php-programmers (and I am not one of them) this might be to simple/redundant so feel free to ignore this.
Cheers Leo