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Framework with a small footprint
CodeIgniter 3 has a 2MB download, including the user guide.
The CodeIgniter User Guide comes with the download. It contains an introduction, tutorial, a number of "how to" guides, and then reference documentation for the components that make up the framework.
Compatibility with standard hosting
CodeIgniter 3 only needs PHP 5.3.7, and plays nicely with almost all shared or dedicated hosting platforms. Many webapps need a database, and CodeIgniter supports the most common, including MySQL.
No restrictive coding rules
Use your own coding and naming conventions, with only a few caveats that deal with classname conflicts. CodeIgniter looks to empower you, not shackle you.
Simple solutions over complexity
CodeIgniter encourages MVC, but does not force it on you.
CodeIgniter consistently outperforms most of its competitors.
No large-scale monolithic libraries
CodeIgniter is not trying to be all things to all people. It is a lean MVC framework, with enough capabilities to improve your productivity, while providing for third-party addons/plugins for additional functionality.
Nearly zero configuration
Much of the CodeIgniter configuration is done by convention, for instance putting models in a "models" folder. There are still a number of configuration options available, through scripts in the "config" folder.
No need for template language
CodeIgniter comes with a simple, substitution based, templating tool. Addons/plugins are available for most of the full-blown templating engines, if that is what you are used to.
Spend more time away from the computer
Don't we all want it? CodeIgniter is easy to learn and to get proficient with.