Sharing a craft/app Folder Across Multiple Sites
This guide is for Craft 2 only. We don’t recommend this approach with Craft 3. Each Craft 3 installation should have its own set of application files.
If you’re running multiple Craft sites off of the same server, it’s possible for them to share the same Craft application files, while each having their own templates, plugins, configs, etc..
The easiest way to do it is by setting the CRAFT_BASE_PATH in your index.php file:
// Path to your craft/ folder
$craftPath = '../craft';
// Override the path to all of the craft/* folders except craft/app/
define('CRAFT_BASE_PATH', __DIR__.'/craft/');
With that one extra line, your index.php file will continue to look for Craft’s app/ folder in ../craft/app/, but all the other craft/\* folders will be expected to be in a separate craft/ subfolder located relative to the index.php file.
Note: If you decide to put your secondary craft/ folder below web root (as we’re doing in this example), it’s up to you to prevent direct HTTP traffic to the folder. You can do that by putting a .htaccess file in that folder with this:
deny from all