Codewalker.com have a tutorial up describing PEAR’s ITX template system. Their forums also have an explanation of how bitshifting operators work in PHP (in the fifth message from the bottom of the thread).
This is PEAR templates and bitshifting by Simon Willison, posted on 18th January 2003.
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Previously hosted at http://simon.incutio.com/archive/2003/01/18/pearTemplatesAndBitshifting