Slim Framework 3 401 response can't set WWW-Authenticate header - rest

Question

Have a REST application using Slim Framework v3. All works as expected, but I cannot seem to set headers for the response.
For example:
$app->any('/[{path:.*}]', function(Request $request, Response $response, $args = null ) use ( $objError, $objDBCon, $objUtil ) {
...
return $response->withAddedHeader( 'WWW-Authenticate', 'API-key realm="restricted"' )
->withJson($apiResults, $httpcode);
});
Works as expected in terms of getting data, getting the correct http status code, etc.
For example I get a the correct response JSON
{ "message": "You must be logged in to access this resource" }
and I get the expected status code:
Request Method:GET
Status Code:401 Unauthorized
and all the standard, correct headers, content-type, etc, etc.
But cannot seem to set any additional headers.
Reference documentaiton https://www.slimframework.com/docs/objects/response.html

Answer 1

My reputation is to low to add a comment:
According to the manual
withAddedHeader method appends the new value to the set of values that already exist for the same header name
Does your header already exists before appending?
I usually create a new header for each response, something like this:
return $response = $next($request, $response)
->withHeader('Access-Control-Allow-Origin', $this->allowedhosts)
->withHeader('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type, Accept, Origin, Authorization')
->withHeader('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS')
->withStatus(200);
hope this helps.