[horde] Installing Horde 4 from Pear results in "Autoloader.php not found" error.

[Ernie Dunbar]

I followed the instructions at
http://www.horde.org/apps/horde/docs/INSTALL#installing-with-pear, and
when I get to the part about completing the configuration, ("You can now
log into Horde without a password") I simply get a page with the following
error:

Warning: require_once(Horde/Autoloader/Default.php)
[function.require-once]: failed to open stream: No such file or directory
in /home/webmail/public_html/horde/lib/core.php on line 44

Fatal error: require_once() [function.require]: Failed opening required
'Horde/Autoloader/Default.php'
(include_path='/home/webmail/public_html/horde/lib:/home/webmail/public_html/horde/pear/php')
in /home/webmail/public_html/horde/lib/core.php on line 44

I figured that the Autoloader module isn't installed, but it is. Running
`pear install horde/horde_autoloader` just tells me that horde_autoloader
is already installed. Searching the drive with 'locate' tells me that it's
in the following paths:

/home/horde/backup/pear/PEAR/Autoloader.php
/home/horde/public_html/pear/PEAR/Autoloader.php
/home/webmail/public_html/horde-1.2.7/pear/php/PEAR/Autoloader.php
/home/webmail/public_html/horde/js/syntaxhighlighter/scripts/shAutoloader.js
/home/webmail/public_html/horde/js/syntaxhighlighter/src/shAutoloader.js
/home/webmail/public_html/horde-old/pear/php/PEAR/Autoloader.php
/home/webmail/public_html/horde/pear/php/PEAR/Autoloader.php
/home/webmail/public_html/horde-webmail-1.1.3/pear/PEAR/Autoloader.php
/home/webmail/public_html/horde-webmail-1.2.7/pear/php/PEAR/Autoloader.php
/home/webmail/public_html/horde-webmail-1.2.9/pear/php/PEAR/Autoloader.php

Evidently, the include_path just isn't set right, but as far as I can
tell, I can't change that in the Horde configuration, and I doubt it would
be wise to change that in the system-wide php.ini configuration.

Why is this broken, and what can I do to fix this?