XML OTA Opentravel - PHP, where to start

Good Evening Experts

Now here is one to consider, I wonder if one of our answers shall be the lucky number 3 Million ;)

I am creating a webservice from scratch and due to our being in the travel industry - and our needs for the webservice being for travel related connectivity - we have decided to go with the Standard @ opentravel.org

I have all the files downloaded and I can understand in theory how they are supposed to work. ie. A Remote Procedure Call from one System to anothers RQ.xml which should return an RS.xml to the system making the call.

I can connect from one domain to another using cURL - which shows the document connected to ie. RQ.xml

What do I do from here? How do I allow only the requestor with the correct POS to have aqccess to the file?, how do I do what needs to be done within the file so that I can then return an RS.xml file? How do I make the file become an RS.xml file from the original RQ.xml

Should I be using XSLT to transform everything with PHP in between? Should I use SimpleXML to traverse the array and run PHP like this?

I am very stuck and would be most grateful and appreciative if you could point me in the right direction and not limit your answers to just one or the other as my questions are above (with my limited knowledge there could be solutions that I dont even know about yet)

Thanks for your Consideration and time.

Best

Brad

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Links for info:
To download the XML Schema: http://www.opentravel.org/Specifications/Default.aspx
Spec I am using: http://www.opentravel.org/Specifications/SchemaIndex.aspx?FolderName=2010B