php rest API POST request returning null - php

Question

I've created a REST API base on this tutorial - note that I am a newbie in php and REST...
Now, I am stuck when calling a POST request. The main return function is as follows:
// Requests from the same server don't have a HTTP_ORIGIN header
if (!array_key_exists('HTTP_ORIGIN', $_SERVER)) {
$_SERVER['HTTP_ORIGIN'] = $_SERVER['SERVER_NAME'];
}
try {
$API = new MyAPI($_REQUEST['request'], $_SERVER['HTTP_ORIGIN']);
$res = $API->processAPI();
echo $res; // contains my json as expected
return $res; // always empty string
} catch (Exception $e) {
echo "Exception: " . json_encode(Array('error' => $e->getMessage()));
}
EDIT
I've just tried something even simpler in the API caller method, namely following:
try {
$res = json_encode(Array('test' => "my message"));
// comment out one or the other to check output...
//echo $res;
return $res;
} catch (Exception $e) {
echo "Exception: " . json_encode(Array('error' => $e->getMessage()));
}
Result with echo is (the way I get responese is below... exact response is between # characters):
#{"test":"my message"}#
Result with return is
##
EDIT 2
Here is how I call the API from C#:
using (HttpClient client = new HttpClient()) {
JObject jo = new JObject();
jo.Add("usr", "username");
jo.Add("pwd", "password");
Uri url = new Uri(string.Format("{0}/{1}", RestUrl, "login"));
StringContent content = new StringContent(jo.ToString(), Encoding.UTF8, "application/json");
HttpResponseMessage response = await client.PostAsync(url, content);
bool isOk = response.StatusCode == System.Net.HttpStatusCode.OK;
// this is where I get the result from...
var res = response.Content.ReadAsStringAsync().Result;
}
I really don't understand this - could someone please explain in non-php expert terms??