Use parse_url and compare host to your local host (often but not always it's the same as $_SERVER['HTTP_HOST']
)
function isexternal($url) {
$components = parse_url($url);
return !empty($components['host']) && strcasecmp($components['host'], 'example.com'); // empty host will indicate url like '/relative.php'
}
Hovewer this will treat www.example.com and example.com as different hosts. If you want all your subdomains to be treated as local links then the function will be somewhat larger:
function isexternal($url) {
$components = parse_url($url);
if ( empty($components['host']) ) return false; // we will treat url like '/relative.php' as relative
if ( strcasecmp($components['host'], 'example.com') === 0 ) return false; // url host looks exactly like the local host
return strrpos(strtolower($components['host']), '.example.com') !== strlen($components['host']) - strlen('.example.com'); // check if the url host is a subdomain
}