[NOTE: Still no Ubuntu in the list of Operating Systems]
I commented about this in the forum but there was no reply there. I tried again on a fresh install so I could document the steps I took.
As mentioned in Bug#788 I worked around the problem of the libapache2-mod-php4 not being installed by default. I then reconfigured php4-mysql and installed php4-imap by hand. I then tried the script installer for Horde and got this result:
Downloading ftp://ftp.horde.org/pub/horde/horde-3.1.4.tar.gz (5262198 bytes) ..
Received 1024 bytes (0 %)
Received 526336 bytes (10 %)
Received 1052672 bytes (20 %)
Received 1579008 bytes (30 %)
Received 2105344 bytes (40 %)
Received 2631680 bytes (50 %)
Received 3158016 bytes (60 %)
Received 3684352 bytes (70 %)
Received 4210688 bytes (80 %)
Received 4736000 bytes (90 %)
Received 5262198 bytes (100 %)
.. download complete.
PHP module imap is required ..
Enabling module in PHP configuration ..
Failed to install script : Missing file to open at virtual_server::../web-lib-funcs.pl line 1917
After trying to do the install again then after the "PHP module imap is required .." message I get "Module is not available even after installation!"