Inspect the referrer in PHP

$referringSite = $_SERVER['HTTP_REFERER']; // is that spelt wrong in PHP ?

$referrerIsValid = false;
$hasReferrer = false;
if (isset($_SERVER['HTTP_REFERER'])) {
    $parts = parse_url($_SERVER['HTTP_REFERER']);
    if (isset($parts['host'])) {
        $hasReferrer = true;
        $referrerIsValid = (bool) preg_match('/(?:^|.)example.com$/', strtolower($parts['host']));
    }
}