I would recommend you break the problem into two pieces:
url: 'get-nodes.php' to
url: 'my-dotnet-url and make the .NET page return static JSON or XML (hardcode the values to the tree)
2) Then find a .NET example that will let you create JSON or XML from wherever you are getting data (I'm guessing probably a database). You just need the output to look like your static data that worked correctly. If you don't know much PHP or .NET learning .NET to get your output correct would be easier than trying to port over that example. If you get stuck, I'd repost a different question and ask how to output the data that results from that static file dynamically and don't have the extjs complication involved!
Hope this helps.