Yes! CodeIgniter is a free PHP framework, licensed with the Open Software License 3.0 (OSL 3.0) and its development is community driven in the public. So in both a legal sense and in the common understanding, CodeIgniter is certified open source software.
You can use the CodeIgniter source code for whatever you’d like, making any changes you like, for essentially any purpose, free or commercial. If you distribute the result of your changes, you must reciprocally license those changes under OSL 3.0, and make the source readily available to the public.
In your distribution, you must retain all copyright and trademark notices in the CodeIgniter files, as well as notices of licensing and other remarks identified in the attribution notice in the source code. Your own work will carry a prominent attribution notice that lets people know that you have modified CodeIgniter, and you must include the full OSL 3.0 license along with the distribution.
No, the application folder of CodeIgniter does not carry the OSL 3.0 license, and your code and any assets always belong to you. The reciprocal licensing and release of a derivative product only applies to changes you might make to the core system files, which are all easily identifiable as they each carry an OSL 3.0 licensing notice in their comment headers. Default files (config, etc.) that are included by EllisLab in the application folder are licensed as AFL 3.0 (Academic Free License 3.0) which allows you to relicense those files to whatever license you have chosen for your code.
You can license your application however you like, but if you happen to distribute a modified derivative of CodeIgniter with your application, you must make those changes and those changes only freely available, licensed with OSL 3.0.
Absolutely, OSL 3.0 licensing only applies to OSL 3.0 licensed code, and does not force itself upon any other code or licenses you may distribute with your application. See the table below for an overview of its compatibility with distribution along side or linking to code covered by other licenses.
|BSD 3 Clause||Yes|
|BSD 2 Clause||Yes|
|Your custom license||Yes|