Hooks concept in codeigniter with simple examples

Hook feature can be useful for adding custom changes without changing the core code.  There may be instances, however, where you'd like to cause some action to take place at a particular stage in the execution process. For example, you might want to run a script right before your controllers get loaded, or right after, or you might want to trigger one of your own scripts in some other location.

Enabling Hooks

The hooks feature can be globally enabled/disabled by setting the 

following item in the application/config/config.php file:

$config['enable_hooks'] = TRUE;

Defining a Hook
Hooks are defined in application/config/hooks.php file. Each hook is specified as an array with this prototype:   

$hook['pre_controller'] = array(
                                'class'    => 'MyClass',
                                'function' => 'Myfunction',
                                'filename' => 'Myclass.php',
                                'filepath' => 'hooks',
                                'params'   => array('bread', 'wine', 'butter')
                                );
CLASS-The class that you wish to invoke, if it is procedural code leave it as blank.
FUNCTION-The function name you wish to call.
FILENAME-The file name containing your class/function.
FILEPATH-Location of the hook file.
PARAMS-Additional parameter if needed it is optional

Hook Points
The following is a list of available hook points.

• pre_system
  Called very early during system execution. Only the benchmark and hooks class have been loaded at this point. No routing or other processes have happened.
• pre_controller
  Called immediately prior to any of your controllers being called. All base classes, routing, and security checks have been done.
• 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() function, used to send the finalized page to the web browser at the end of system execution. This permits you to use your own display methodology. Note that you will need to reference the CI superobject with $this->CI =& get_instance() and then the finalized data will be available by calling $this->CI->output->get_output()
• cache_override
  Enables you to call your own function instead of the _display_cache() function in the output class. 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.