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However, I try answering you:
You say, you already know XML syntax, PHP actually pulls latest information (records) from db and dumps that in XML structure (It's really easy; if you know what's XML structure/syntax is..); this file often named 'rss.php' can be syndicated remotely and obviously with latest information. I know just this.
PHP is for making XML document dynamic..
XML is for easy data sharing and data transport over the Web..
May be, let's wait some seniors to comment :)