如何在PHP Get Request上设置内容类型？

Sorry if this is really simple but I am new to REST requests. I am trying to make a GET request to a REST API I have been provided with but I am getting a 415 error (Unsupported Media Type).

Here is my (very basic) code :

<?php

header('Content-Type: application/json;charset=utf-8;');

$url = "https://opsview.somedomain.net/rest";
$response = file_get_contents($url);
echo $response;

?>

The API I am connected to says it supports json or xml so not sure why I am getting this error? Any ideas?

Any help will be much appreciated.