This is used to implement the --installroot option for installation. In earlier PEAR versions, this was implemented in PEAR_Installer::install(), but this makes it more difficult to track with the introduction of channels, and to satisfy better encapsulation, it has been moved to PEAR_Config.
Pass in a full path. On retrieving any directory configuration variable, the value will be prepended with the installroot specified in this method. For example, if php_dir is /usr/lib/php, and setInstallRoot() is used with /hoopla/boo, the value returned from get() would be /hoopla/boo/usr/lib/php. Use an empty string '' to reset an installroot.
installation directory to prepend to all _dir variables, or FALSE to disable