[imp] htmlspecialchars: US-ASCII not supported warning in imp

[Ethan Tuttle]

Hi, I'm a happy IMP user but I'm having a little trouble now that I've
upgraded to php 4.3 under apache 2.0.43.

I'm running RELENG_3 of imp and RELENG_2 of horde, checked out today.  PHP
is version 4.3.0RC4.  I tweaked my pear installation so that IMP and horde
are happy enough with it.

When I view a message whose charset is identified as US-ASCII in IMP, I get
a warning like this at the top of the window:

Warning: htmlspecialchars() [function.htmlspecialchars]: charset `US-ASCII'
not supported, assuming iso-8859-1 in
/var/www/horde/imp/lib/MIME/Viewer/text.php on line 44

I see in the php release notes:

Fixed bug #18521 (htmlentities didn't warn about unsupported charsets).
(Wez)

I'm guessing that my bleeding edge php install is spitting out this new
warning.

I searched the archives / bug databases but didn't come up with a solution.
I hope this hasn't been answered before.

Is this a problem with my PHP install?  I couldn't find any references to
adding US-ASCII support to htmlspecialchars for my php installation.

Has this problem been addressed in imp HEAD?  I browsed the cvs briefly but
didn't see any indication that it has.

Is there a work around for those of us running RELENG_3 with PHP 4.3?

Thanks much!

Ethan