When you open a php-file with nvu, you get the typically save/openwith-dialog you would also get in Firefox if you click on a link to a php-file.
So you cannot open a php-file at all.
The problem is also described here: http://forum.nvudev.org/viewtopic.php?t=3189
There is also a hack for the problem as an extension (but it doesn't work for me): http://fabiwan.kenobi.free.fr/HandCoder/
Sorry, this is completely beyond Gentoo scope to get fixed. Try your luck upstream.
It seems that this project is dead. At least I read some texts in the forum which says that. The developers haven't answered on this (and it seems that they aren't realy active anymore at all).
This bug is already known for quite a long time (2 years) and the developers seems to ignore it.
Also, the forum is locked for new messages at the moment. That was the usual place for nvu to report bugs.
I will perhaps try to get in contact directly with the developers and ask for further information.
Btw., here is another workaround: http://www.von-ruetzen.de/linux/nvu.php
(In reply to comment #2)
Well, the workarounds are kinda useless because nvu insists on adding HTML tags to PHP files, so... I guess you'd be better off finding yourself a different editor for PHP code.
I don't realy want to edit the php-code (that is only the minor part of the site). The site itself for me is more or less completly HTML besides on some rare position some php-tags which can easily be ignored. NVU works realy great (and provides me exactly with the features I want to have) if I just rename the file with an html-extension and edit it then with NVU and rename it back after.
OK, my idea now was to fork this. It seems that this already was done: http://kompozer.net/
There is even already also an bug request for an ebuild for it: bug 146761
I have checked out KompoZer now. Sadly the bug is still in there. But now I will try to contact the KompoZer developer.