$context  = stream_context_create(['http' => ['header' => 'Accept: application/xml']]);
$url = 'https://my.url/asdfasdf/webrecruitment/webrecruit/rss/random_rss.xml';
$xml = file_get_contents($url, false, $context);
$vacancies = simplexml_load_string($xml);
var_dump($vacancies);

Warning: file_get_contents(https://my.url/asdfasdf/webrecruitment/webrecruit/rss/random_rss.xml): failed to open stream: HTTP request failed! HTTP/1.1 406 Not Acceptable in ....