The stdobject of PHP is basically a class object. So, whenever you know the structure of XML message performed the stdobject, you will simply call it such as following: stdobject->property_name. So for example, if the stdobject name is $product, and you need to retrieve the keywords, simply call $product->keywords, thats all. And in case, the keywords is an object also, you could use the same method for accessing its properties.
Thats all.
So for your project, simply call PHPSimpleXML object for loading the XML content, and parse the message such as my description above.