PHP: how to check if the client is local?

I need to check if a file is opened "locally" (same machine or network). I'm using:

<?php
if ((substr($_SERVER['REMOTE_ADDR'],0,8) == "192.168.") || ($_SERVER['REMOTE_ADDR'] == "127.0.0.1")) {
    // client is local
} else {
    // client is not local
}

But I'm not sure this is the best way.

What is a more foolproof way of doing this?