See the difference between this:

<!-- Login.php Customer Connection login -->

<?php header("Location: PrincipalHome.html"); ?>

and this?

<?php header("Location: PrincipalHome.html"); ?>

No comment line! Well, bunkie -- guess what? Some Apache varieties apparently think that HTML comment line is, in fact, a header start line. Yank it. Maybe I'll go back and retrofit PHP comments into the straight-up PHP sources. I probably makes more sense that way anyway. I guess.

I just hate finding out this kind of stuff The Hard Way®.