Working for OP:
$host = parse_url($url, PHP_URL_HOST);
The version of PHP I have to work with doesn't accept two parameters (Zend Engine 1.3.0). Whatever. Here's the working code for me - you do have to have the full URL including the scheme (http://). If you can safely assume that the scheme is http:// (and not https:// or something else), you could just prepend that to get what you need.
Working for me:
$url = 'http://foo.com/bar/foo';
$parts = parse_url($url);
$host = $parts['host'];
echo "The host is $host\n";