You need to parse the url first, so it goes like this:
$url = "https://www.example.com/profile#picture";
$fragment = parse_url($url,PHP_URL_FRAGMENT); //this variable holds the value - 'picture'
If you need to parse the actual url of the current browser, you need to request to call the server.
$url = $_SERVER["REQUEST_URI"];
$fragment = parse_url($url,PHP_URL_FRAGMENT); //this variable holds the value - 'picture'