Somtimes it may be required to host several distinct ILIAS installations in one apache site. This can be done fairly easy if your ILIAS- installations can be accessed with different urls like

"www.myilias.de/ilias1"
"www.myilias.de/ilias2"
(...)

In that case you can generate several subdiretories in your document-root and install each ILIAS there.

Sometimes you may want your productive ILIAS installation to be accessible through a top- url and some other installations to a url below this top-url:

"www.myilias.de"
"www.myilias.de/testilias"

to have a testing or staging environment separate from your productive environment.

At this point, it is not advisable to install the test-ilias in a newly created subdirectory on your server, since lateron updates or client installations may fail for several reasons.

To achieve this setting, create a new directory on your server which is not located below your ILIAS directory. For the example above use:

mkdir /var/www/testilias

to host the new installation's files.

In the apache site configuration use the following statement:

(...)

Alias "/testilias" "/var/www/testilias"

(...)

to instruct apache to redirect the access for the sub-site to the new installation.
Now you can fully install ILIAS with the usual procedure to the new directory

This procedure was tested using apache 2.4 and ILIAS 5.2 (beta) in january 2017