C# HTTP Body with GET method

What does one call an API that actively goes against REST? "HASTE"? "DISQUIET"?

With a bit of luck they service just doesn't care what the verb is and the PHP code was just happening to use GET and hit the bug that the server didn't block it which is a pretty minor bug as long as it behaves correctly, and it'll be fine with POST.

Failing that, your best bet is to see if they have an alternative method that either (if it's a reading request that naturally fits GET) accepts parameters in the URI with perhaps appropriate headers being used as per RFC 2616, or else can accept something through POST, GET etc.

If that doesn't work, you'll have to build an HTTP client on top of TcpClient. Which would be pretty horrible.

It is entirely possible, but you have to use the newer HttpClient class: https://stackoverflow.com/a/47902348/70345