Adding CodeIgniter to an existing site

[eluser]Pascal Kriete[/eluser]
If you open up your index.php file you'll see that it specifies two paths. One for the system folder, and one for the application folder (which you can rename, btw).
All your sites can share one system folder, located anywhere on your system. For example mine reads:
/Applications/MAMP/CI/system

For you application folder you can do the same thing, but obviously every application will need it's own index.php file (you will probably rename them as well) and usually it's own application folder. Again, here's the basic path for mine:
/Applications/MAMP/CI/apps/<appname>