what is hooks & How many types of hooks?

CodeIgniter’s Hooks feature provides a way to modify the inner workings or functionality of the framework without touching the core files.

The following types of hooks:

• pre_system Called very early during system execution.
• pre_controller Called immediately prior to any of your controllers being called.
• post_controller_constructor Called immediately after your controller is instantiated, but prior to any method calls happening.
• post_controller Called immediately after your controller is fully executed.
• display_override Overrides the _display() method.
• cache_override Enables you to call your own method instead of the _display_cache() method in the Output Library. This permits you to use your own cache display mechanism.
• post_system Called after the final rendered page is sent to the browser, at the end of system execution after the finalized data is sent to the browser.