Thats no error, just a notice, turn off Error-Reporting in PHP or decrease the reporting level to errors only...or...edit the files by hand and add a @ before the commands to turn off showing errors.

You'll always get strange problems (images not showing correctly without copying them around, and so on) with modifying sth., I did it like this:

1. In the webroot, do:
# ln -s . ./horde

2. In the Apache VHost-Config-File do:
(in the VirtualHost-Section, add)
<Directory /path/to/your/horde/root ...