I'm just documenting this, after trying to install VTcalendar in an i-bay, and finding I needed to enable PEAR for that i-bay first. It seems that PEAR is available for horde, but is disabled for all i-bays by default, but can easily be enabled.
From the command line, it looks like this (where {ibay} is the name of your i-bay):
# db accounts setprop {ibay} PHPBaseDir /home/e-smith/files/ibays/{ibay}/:/usr/share/pear-addons:/usr/share/pear
#signal-event ibay-modify {ibay}
You can see the current settings using this command:
db accounts show {ibay}
Hope that helps someone.[/code]