several sites sharing the same script

Hello all, I am writing new php application which will be used by several sites. All sites will use same script (all files), but every will have its own config.php file. I would like to place all necessary files to separate directory and all sites should share them, so all sites will be updated. for e.x. structure will be: | - script directory |- site1.com dir config.php |- site2.com dir config.php |- site3.com dir config.php Does anyone have idea how to organize and share files? Thanks For Your Time !