If you're running the web server on a Unix box that supports PAM, look into the nsswitch modules that allow a local Unix user to authenticate against various directory services. Then look for the php pam auth class that is floating around out there.
I wouldn't recommend this approach unless the webserver is behind your firewall and inaccessible from the outside.
Note that this type of thing is a complicated mess and isn't very well documented for the faint of heart (non-unix guru), so if PAM and nsswitch are new terms, go with Paul's recommendation....