We were able to get Kerberos working with Apache using mod_auth_kerb. I've then been able to authenticate users using Apache's authentication and been able to get their Kerberos username and password from the Apache environmental variable REMOTE_USER and the PHP environmental variable PHP_AUTH_PW.
We've successfully implemented Kerberos for internal users. However, we haven't implemented any Kerberos authentication for our external customers. For the ease of administration, we've stuck to using a MySQL database for our external user accounts.
Hope this is helpful.