Use phpinfo() and test see what you can use to identify people.
I'm not sure, but it can be $REMOTE_URL.
You can't test their domain name.
As if founders of phpbuilder would come from phpbuilder.com
What is it you want to do??
If - the users you want to grant access - comes from a secure intranet, you can use HTTP_REFERER
For example.
if ($HTTP_REFERER == "www.myoffice.com/intranet/page.html") {
header("location: the_secret_page.php");
} else {
echo '<input name=password>....';
}
OR eregi the $HTTP_REFERER for "www.myoffice.com/intranet".