PHP $_SERVER[‘HTTP_HOST’] vs. $_SERVER[‘SERVER_NAME’], am I understanding the man pages correctly?

That’s probably everyone’s first thought. But it’s a little bit more difficult. See Chris Shiflett’s article SERVER_NAME Versus HTTP_HOST.

It seems that there is no silver bullet. Only when you force Apache to use the canonical name you will always get the right server name with SERVER_NAME.

So you either go with that or you check the host name against a white list:

$allowed_hosts = array('foo.example.com', 'bar.example.com');
if (!isset($_SERVER['HTTP_HOST']) || !in_array($_SERVER['HTTP_HOST'], $allowed_hosts)) {
    header($_SERVER['SERVER_PROTOCOL'].' 400 Bad Request');
    exit;
}