Those eagle-eyed developers who have been tracking the Elgg core SVN may have noticed that I have recently committed a bunch of captcha related changes, including a simple captcha module.

I just thought I’d write a quick post about it as this module makes use of a bit of Elgg functionality which has been around for a while, but that I know a number of plugin developers have missed.

Namely, the ability to extend actions.

When the Elgg framework calls an action the Action handler triggers a plugin hook called “action” before executing the action itself. This hook looks like this:

$event_result = true;
$event_result = trigger_plugin_hook('action', $action, null, $event_result);

Where $action is the action being called. If the hook returns false then the main action will not be executed.

The captcha module uses this to intercept the register and user/requestnewpassword actions and redirect them to a function which checks the captcha code. This check returns true if valid or false if not (which prevents the associated action from executing).

This is done as follows:

register_plugin_hook("action", "register", "captcha_verify_action_hook");
register_plugin_hook("action", "user/requestnewpassword", "captcha_verify_action_hook");


function captcha_verify_action_hook($hook, $entity_type, $returnvalue, $params)
$token = get_input('captcha_token');
$input = get_input('captcha_input');

if (($token) && (captcha_verify_captcha($input, $token)))
return true;


return false;

As you can see, this lets a plugin extend an existing action without the need to replace the action itself. In the case of the captcha plugin it allows the plugin to provide captcha support in a very loosely coupled way.

Happy coding!

4 thoughts on “Extending actions in Elgg

  1. good explanation. i need to know one thing. what if we would like to redirect on our own selected page after generating the error if captcha not matched.?
    i use this captcha on above mentioned site. it works fine but error showing on dashboard except same page.

  2. The default captcha plugin in Elgg 1.7.11 does not work – when enabled, it produces a broken image above the text box for captcha input. It appears the token is generated fine by viewing the HTML source; however, the image does not display. I’ve seen many complaints about this, but no fix.

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