The problem is that you have version 5.5, and the HowTo has not yet been updated for 5.5. All the information can be found from searching these forums, but here is what I think you need. (by this I mean it worked for me, but I am sure that there is a better way)
For the PHP 4.2.2 (I don't know if this is essential - so you could try to skip this)
(Also I had to remove php-pgsl to avoid errors, however I am sure that there is a better way. "rpm -e php-pgsql.rpm" )
Install Dan Brown's RPMs
php-4.2.2-1db_rh72.i386.rpm
php-mysql-4.2.2-1db_rh72.i386.rpm
php-imap-4.2.2-1db_rh72.i386.rpm
php-ldap-4.2.2-1db_rh72.i386.rpm
Note: Webmail no longer works (blank screen after log-on), so to restore it:
Download Log-1[1].2.tar from
http://pear.php.net/package-info.php?pacid=8 # tar -xvf Log-1[1].2.tar
# /bin/cp -R Log-1[1].2.tar/* /usr/bin/share/pear
Now Webmail will be working again.
For the newer IMP/Horde etc you need only 3 RPMs from Dan Brown.
e-smith-turba-1.1dcb-1.noarch.rpm
horde-2.1-1es.noarch.rpm
imp-3.1-2es.noarch.rpm
Then
/sbin/e-smith/expand-template /home/httpd/html/horde/imp/config/servers.php
/sbin/e-smith/expand-template /home/httpd/html/horde/imp/config/conf.php
And Finally I had to 'save' the webmail settings in server manager before it would work (even though I left it at enabled).
This worked for me on both a fresh install, and a upgrade from 5.1.2.