Hi Sharma
Well, OK, but WDDX is just a form of XML. It's a tree of tags, just like any other XML. What the WDDX package does is no different from parsing RSS or WSDL or any other form of XML. You look for the tags you want and vacuum up the content until you get to the end tag. You then tell PHP what to do with the content, e.g. stick it in an array or convert it straight to HTML or whatever.
The WDDX module is doing exactly that - it's just written the code for you already and given you a single command. If you want to be completely flexible so that you can parse any kind of XML, it's good to know how the code actually works. You may also find the new simpleXML module in PHP5 useful. If you really want to do cool things with XML, you should also look into XSLT. XSLT isn't easy, but it's powerful and lets you do what you want with any dialect of XML.
HTH
Norm