yo, I'm doing exactly this myself. basically I wrote an application framework w/ pluggable database abstraction, LDAP server abstraction, and authentication abstraction. it sucks tho cuz I can't find a krb5 implementation for php... I found 2 extremely sketchy unix/linux krb5 php libraries that are waaaay simplistic, one is here:
http://bigworm.colorado.edu/phpkrb5/
... but nothing with precompiled windows binaries or any sort of decent krb api, sad to say. yeah if you're interested in the ldap stuff I can help you and I would appreciate any help w/ this krb thing. thanks!
-fish