[imp] Problem with mysql sessions

[Data Vortex]

Greetings,

I am trying to install Horde 2.2 / IMP 3.1 on a fairly out-of-box Red Hat
8.0 installation.  PHP is 4.2.2 and PEAR is 1.0.1.  Apache 2.0.40 and mysql
3.23.54a.

This installation has not yet been functional.  This is my first time trying
to install IMP with mysql sessions.  I installed from tarball.

I configured horde to let IMP handle authentication, which auths against
IMAP.  I know authentication is working because I have lots of:

Jan 23 02:06:32 not imapd[27585]: Authenticated user=datavortex
host=localhost.localdomain [127.0.0.1]
Jan 23 02:06:32 not imapd[27585]: Logout user=datavortex
host=localhost.localdomain [127.0.0.1]

However, every time I try to login, I immediately get this error from
redirect.php:
----------------------------------------------------------------------
Notice: Undefined index: socket in /usr/share/pear/DB/mysql.php on line 108

Warning: Cannot add header information - headers already sent by (output
started at /usr/share/pear/DB/mysql.php:108) in /www/
mail/horde/imp/redirect.php on line 84
----------------------------------------------------------------------

I set my log level to DEBUG to investigate further and I see lots of failed
attempts to record sessions in the database.  Checking in the datebase, I
can confirm that despite the successful logins, no sessions were recorded:

Jan 23 02:06:25 not HORDE[27564]: [imp] SQL Query by
SessionHandler_mysql::read(): query = "SELECT session_data FROM
horde_sessionhandler WHERE session_id = af2eb3a860b09ffca88f4fa61a18c3f8"
[on line 96 of "/www/mail/horde/lib/SessionHandler/mysql.php"]
Jan 23 02:06:25 not HORDE[27564]: [imp] Error retrieving session data (id =
af2eb3a860b09ffca88f4fa61a18c3f8) [on line 104 of
"/www/mail/horde/lib/SessionHandler/mysql.php"]

In horde/config/horde.php I have set:
$conf['sessionhandler']['type'] = 'mysql';
$conf['sessionhandler']['params'] = array();
$conf['sessionhandler']['params']['phptype'] = 'mysql';
$conf['sessionhandler']['params']['hostspec'] = 'localhost';
$conf['sessionhandler']['params']['username'] = 'horde';
$conf['sessionhandler']['params']['password'] = 'myhordepassword';
$conf['sessionhandler']['params']['database'] = 'horde';
$conf['sessionhandler']['params']['table'] = 'horde_sessionhandler';

I used sessionhandler.sql to create my database table and the GRANT needed,
and manually verified that it is correct.  In php.ini I have set:
mysql.default_socket = /var/lib/mysql/mysql.sock

What have I overlooked?

Thanks very much for your help,
DV