CodeIgniter
Open Source PHP Framework (originally from EllisLab)
...The framework provides convenience libraries for sessions, form validation, encryption, email, file uploads, and more, all optional so the footprint stays small. Because it doesn’t force heavy conventions, CodeIgniter is flexible for both small projects and larger modular applications where you may want to craft your own architecture. Its documentation and clarity of code make the learning curve gentle for PHP newcomers. In production, CodeIgniter’s efficiency and minimal overhead make it a steady option when performance and stability are priorities.