At some point of time, we always need to tweak or customize the core functionality or flow of the application.

If you are aware of WordPress, then there are hooks and filters for this.

In moodle, We can extend the core features and even tweak them through events and callback functions, even without making any change in the core files.

What is moodle EVENT ?

Moodle events, are the hooks, on which you can register your functions to be called. there are many-many core events available .

you need to create a plugin , under db folder , create an observer.php file.

put following code.

// create a class name as core_event_sample_observer and write a function observe_all, function would be called as soon as user is created in system.

$observers = array(
    array(
          'eventname'   => '\core\event\user_created',
        'callback'    => 'core_event_sample_observer::observe_user',        'includefile' => null,
        'internal'    => true,
        'priority'    => 9999,
    ),

);

You can include your class by putting file path . and you can get the user detail , by defining parameters in a function.

class core_event_sample_observer {

public static function observe_user($event){
    $userid = $event->objectid; // created user id
    print_R($event);
// write your code here and do the needful operation
   }
}

An administrator can view a list of all types of events from Site administration > Reports > Events list.

more information can be grabbed form here,

https://docs.moodle.org/dev/Events_API#Events_API

You can execute the code in events callback but think like this as a background process, as it will not interact with page. Simply, you can not use redirect or you can not put any thing that you want to be a part of the rendereded page. For such cases, there are callbacks.

Extending through Callbacks

To solve the above challenges, we use the callback.

Callbacks are the function, in which moodles try to call during execution. These are hooks as in wordpress.

• You write the function in lib.php of your plugin in a standard format.
• The function will be called during execution. Moodle will try to check the function by including files from all the plugins.

following is the sample.

// assuming , xyz is a local plugin and callback will be done while code is being executed from standard_footer_html.

// this will include this js at footer of the moodle page.

function local_xyz_standard_footer_html() {
global $CFG, $DB, $USER;
   
    $output = <<<STR
    <script>
   
    window.onload = function(){
        alert("Hello");
    }();
    </script>
STR;
    return $output;
}

• as you are writing this, so you do not need to customize and moodle file to inject your code into the page .
• There are many callbacks already available for core functionalities like page rendering or user login or user registration etc .

List of one-to-many callbacks

List of one-to-one callbacks