Switch PHP-CLI from PHP 4 to PHP 5:

root@server.example.net:/usr/local/bin# rm php 
root@server.example.net:/usr/local/bin# ln -s /usr/bin/php5 php

Give access to initial configuraton page:

/etc/horde/horde3/conf.php

//echo "Horde3 configuration disabled by default because the administration/install wizard gives the whole world too much access to the system."; 
if ($_SERVER['REMOTE_ADDR'] != 'your.ip.address.here') 
exit (0);

Install database upgrades:

mysql -u{user} -p{pass} horde3 < examples/scripts/upgrades/3.0_to_3.1.mysql.sql 
mysql -u{user} -p{pass} horde3 < examples/scripts/upgrades/3.1_to_3.2.mysql.sql

Fix database upgrade script:

/usr/share/doc/horde3/examples/scripts/upgrades/move_history_out_of_datatree.php

//@define('HORDE_BASE', dirname(__FILE__) . '/../..'); 
@define('HORDE_BASE', '/usr/share/horde3');

Run database upgrade script:

root@server.example.net:/usr/share/doc/horde3# php examples/scripts/upgrades/move_history_out_of_datatree.php

Configure the Horde instance
e.g. https://www.example.net/horde3/

Protect test scripts - see also /usr/share/doc/horde3/README.Debian

/etc/horde/apache.conf

<Files test.php> 
    deny from all 
</Files>