Templates are what are generally recommended to really separate presentation from coding. If you go here:
http://www.phpbuilder.com/columns/
and scroll down, you'll see several articles on templating approaches.
That being said, I think true separation is rarely achieved because of conditionals, loops and other flow control constructs that are so handy to use in PHP. The key I guess is to minimize these and call out to functions or methods that are required/included so that the HTML is as uncluttered as possible.
Dave
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