[horde] Login (su) authentication fails with Apache error "su: must be run from a terminal"

[Rob Eyre]

I'm trying to get Horde to authenticate local users on my Gentoo box
running Horde 3.1.1.  I've tried Login (su) authentication but the
apache error_log shows "su: must be run from a terminal".  I also tried
pam authentication but that gave me a php warning saying that the dll
wasn't found (even though I changed it to something I know is in
/etc/pam.d and works).

This is a fresh install which I'm trying to get off the ground.  Any
help much appreciated.

Apache 2.0.58
PHP 5.1.4-r4
Horde 3.1.1-r1
Running http://myhost/horde/test.php checks out fine
Cookies set for /horde
Using Apache virtual hosting

-Rob