You can just use HttpWebRequest or WebClient to make web requests like you would have with CURL in your PHP client...
If you need to deal with JSON based responses, JSON.Net is a fantastic library.
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://a/rest/uri");
request.Method = "POST";
request.Headers.Add("Authorization: OAuth " + accessToken);
string postData = string.Format("param1=something¶m2=something_else");
byte[] data = Encoding.UTF8.GetBytes(postData);
request.ContentType = "application/x-www-form-urlencoded";
request.Accept = "application/json";
request.ContentLength = data.Length;
using (Stream requestStream = request.GetRequestStream())
{
requestStream.Write(data, 0, data.Length);
}
try
{
using(WebResponse response = request.GetResponse())
{
// Do something with response
}
}
catch (WebException ex)
{
// Handle error
}