Observer/Notifier System

About notifiers and observers


One of the many goals of the Zen Cart project has always been to make it simple for third party developers to add functionality to the core code in an easy and unobtrusive manner. To do this we use the override system, the auto inclusion system and the observer/notifier system.

The observer/notifier system is an implementation of the “pub-sub” pattern that was introduced to give developers deep access to core operation without the need to touch any core files at all. Although the implementation was written for an object-oriented code base, it can also be used with procedural code.

At a high level, a developer will do the following:

  • identify an event that they want to monitor, such as when a customer has just successfully logged in.
  • register an observer to be notified when that event occurs.

If/when that event occurs, the base class receives control and looks to see if any registrations exist for the event. If so, all registered observer-classes are called, and they perform their custom actions.

Here are some ‘quick links’ to various sections of this documentation:

  1. Triggering Event Notifications. Identifies the mechanism used to trigger a notification.

  2. Observing Notifications. Identifies the mechanisms used by event observers to perform their customizations.

  3. Loading Your Observer Class. Identifies how to load your observer-class so that it can begin its observations. You may choose between automatic loading and manual loading.

  4. Advanced Topics. This section identifies additional methods that can be used to auto-load an observer-class and create event-specific event-handlers.

    a. Event-Specific Update Methods.

    b. Generic Formal Parameter Interpretation.

  5. Additional Information. This section has references to additional documentation on the observer/notifier system.

  6. Complete Examples This section shows examples of using notifiers and observers.

Triggering Event Notifications

The point of the observer/notifier system is to enable developers to write code that listens for certain events to happen and then perform a customized action at that point in the code execution flow.

Event names are strings and are triggered via a notify function. You can see a list of notifiers in the Zen Cart core code for reference.

Note: Plugins (Zen Cart extensions) can also trigger notifier events.

The notify method takes the following inputs:

Input Name Required? Description
$eventId Yes The string ’name’ of the event, e.g. NOTIFIER_CART_ADD_CART_END.
$param1 No A read-only variable, the format of which varies by the $eventId. Defaults to an empty array.
$param2 … $param9 No A collection of read-write variables, passed as a reference. The code that passes these variables is giving permission for an observer to update these variables. Each variable’s format (and presence) varies by the $eventId and defaults to null.

Class-Based Event Notifications

Within a class that extends the Zen Cart base class, e.g.:

class shopping_cart extends base

… events can be triggered via the $this keyword:


For example, the shopping cart class triggers this event after an item has been added to the cart:


When a class triggers a notification using the $this keyword, all public class-variables are available for use by a watching observer. If a class desires to trigger a notification without that access, the global $zco_notifier can be used to trigger that notification.

Procedural Event Notifications

In procedural application code (actually, in any code that is not inside a class which extends base), use the global $zco_notifier object to trigger events.

For example, the zen_mail function triggers the following event, which allows a plugin to update the to-be-sent email format:

$zco_notifier->notify('NOTIFY_EMAIL_DETERMINING_EMAIL_FORMAT', $to_email_address, $customers_email_format, $module);

Observing Notifications

Writing Observer Classes

To take advantage of notifiers, developers need to write classes to watch for them, i.e. observe. There’s even a nice directory, includes/classes/observers and admin/includes/classes/observers, where developers can put these classes.

There are three base-class methods that observers use to provide their custom actions:

Method Name Description
attach This method identifies a list of event(s) that the observer-class is interested in monitoring.
update This method is the call-back when an event ‘fires’.
detach This method identifies a list of event(s) that the observer-class is no longer interested in monitoring.


The attach method is used by an event observer to ‘register’ to receive control when a specified event (or list of events) occurs. This method takes two parameters:

  1. $observer. Identifies ‘who’ is requesting to receive control, i.e. $this which identifies the current class. This value is used by the base class to create a unique ID associated with the observation request (just used internally, you don’t need to worry about that unique ID).
  2. $eventIDArray. A simple array of event names that the observer-class is listening for (‘observing’).

For example, /includes/classes/observers/class.products_viewed_counter.php requests to be notified whenever the event NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR is triggered by attaching to that event. That event is then triggered in application code by the various product types’ main_template_vars.php modules, causing the observer class to respond as described in the update section below.

class products_viewed_counter extends base {

  function __construct() {
    $this->attach($this, array('NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR'));


The update method is used by an event observer to perform event-specific actions when an event is triggered.
Note: If you find update is not being triggered by an event as expected, ensure your class is being instantiated before the notifier event. Auto-loaded observers are loaded at breakpoint 175. For earlier instantiation, the observer must be manually loaded, see Choosing When to Load an Observer.

The update method may be passed up to 11 parameters:

  1. &$callingClass. This is a reference to the class in which the event occurred. If the event is triggered by a class other than the base (e.g. the order or shopping_cart class), then this variable can be used to manipulate any public properties within the calling class. (eg: if the event is called from the order class, then inside the observer you would refer to the order class’s $this->info property using $class->info)
  2. $eventID. The name of the event triggered.
  3. $param1. Read-only data. The value is dependent on the $eventID. Sometimes named $paramsArray if it’s an array rather than a single value.
  4. &$param2 through &$param9. Updateable variables provided by the code triggering the event. These values are dependent on the $eventID.

Note: Instead of the generic update() method that fires irrespective of which eventID was triggered, you can also choose to use event-specific update-methods to handle event-related processing.

Here’s the full implementation for /includes/classes/observers/class.products_viewed_counter.php:

class products_viewed_counter extends base {

  function __construct() {
    $this->attach($this, array('NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR'));

  function update(&$class, $eventID, $paramsArray = array())
        global $db;
                SET   products_viewed = products_viewed+1
                WHERE products_id = " . (int)$paramsArray . "
                AND   language_id = " . (int)$_SESSION['languages_id'];
        $res = $db->Execute($sql);


When the NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR event is triggered (and the shown defined constant is set to ‘on’), the $paramsArray parameter is expected to be an integer value that identifies the specific product to be updated. The observer’s update method thus casts that $paramsArray parameter to an integer value and performs the products_viewed update.


The detach method is used by an event observer to ‘un-register’ from receiving specified event (or list of events) notifications. This method takes two parameters:

  1. $observer. Identifies ‘who’ is requesting to receive control, i.e. $this which identifies the current class. This value should be the same as that used to attach to the no-longer-wanted event.
  2. $eventIDArray. A simple array of event names that the observer-class is no longer ‘interested in’.

While seldom used, here’s one way it might be used: If, for instance, an observer was interested in the first triggering of a given notification, the observer’s update method, upon receiving that notification, could trigger the associated detach so listening stops.

Loading Your Observer-Class

Observers may be loaded automatically or manually. Automatic loading is less work for plugin authors, but offers less flexibility in terms of load-order which can sometimes interfere with required dependencies.

Choosing “When” to Load an Observer

If your observer-class performs actions prior to the page-specific loading (e.g. monitoring for cart-related actions), you’ll need to make sure that your observer is loaded and instantiated before any watched-for notifier is triggered in application code. In these cases, review the base Zen Cart auto-loader ([/admin]/includes/auto_loaders/config.core.php) to identify the load-point required.

Auto-loaded Observers

If you’re developing a plugin that uses an Observer class you might wish to utilize auto loading and instantiating of observers. These auto-instantiated Observers will be loaded near the end of the list of load-points, so typically all dependencies will already be available. This works great in the vast majority of cases, and makes your Observer less brittle and easier to maintain across Zen Cart version upgrades.

By following a naming convention Zen Cart will both load and instantiate your Observer class automatically. Here are the requirements:

  1. The file is in the /includes/classes/observers sub-directory and named like: auto.your_plugin.php. Note the auto. prefix. All files in this directory that start with auto. will be included (i.e. loaded).
  2. The file defines a class named zcObserver + the CamelCased filename with no underscores. For example, a file named auto.your_plugin.php will contain a class named zcObserverYourPlugin. (For debugging assistance, a myDEBUG*.log file will be generated if a properly-named file is loaded, but the class name doesn’t conform to these rules.)
   class zcObserverYourPlugin extends base { 

Note: This technique will work so long as your class doesn’t have any special requirements on its load point (auto-loaded classes are loaded at point 175, after all other system dependencies are loaded). Most observers won’t need to be loaded “before” all other regular dependencies, so load-point 175 is fine in most cases. If your observer needs to be loaded earlier, then don’t use this special naming convention, but instead manual loading, which is described below. An example of a notifier whose observer should not use auto loading is ZEN_GET_PRODUCTS_STOCK. This fires from the shopping cart class, which has a load point of 80. If you need to observe this notifier, you will want to load earlier than 80.

Note: Auto loading has been available on the storefront side since Zen Cart 1.5.3, and on the admin side since Zen Cart 1.5.7. For versions prior to that you can use the Manually-loaded option below. Alternately, to backport admin side autoloading to 1.5.6, see this PR.

Changing a Manually-loaded Observer to use Auto Loading

The Products Viewed Counter described above could provide the same functionality and not need its auto-loader component if the observer-class file was renamed to /includes/classes/observers/auto.products_viewed_counter.php and its class name was updated to be zcObserverProductsViewedCounter.php.

Manually-loaded Observers

For manually-instantiating Observers at customized load-points, you must provide at least two files in your plugin’s distribution to get that class loaded and instantiated:

  1. /includes/auto_loaders/config.your_plugin.php
  2. /includes/classes/observers/class.your_plugin.php

To load and create an ‘instance’ of your observer-class so that it is operational, you’ll create a file in the Zen Cart /includes/auto_loaders (or /admin/includes/auto_loaders) sub-directory to perform those tasks.

For the example used above, that was the file named /includes/auto_loaders/config.your_plugin.php.

The file contains two auto-load statements:

  1. An autoType of class to load your observer’s class-file.
  2. An autoType of classInstantiate to create an instance of your observer-class.

The ’load-point’, in this example 200 indicates the relative position within the auto-loading process at which the auto-load actions are to be performed. Most observers can safely load at load-point 200 or later (after all the base Zen Cart auto-loaders have completed). See Choosing When to Load an Observer for some special cases.

if (!defined('IS_ADMIN_FLAG')) {
 die('Illegal Access');
$autoLoadConfig[200][] = [

    // the filename, relative to the `classes` folder:
$autoLoadConfig[200][] = [

    // the name of the class as declared inside the observer class file

    // the name of the global object into which the class is instantiated

Advanced Topics

Shared Logic For Multiple Events

Given that the Observer class is indeed a class, you can break out your processing logic into many smaller discrete functions for individual purposes. This allows you to call that private/protected function from within multiple event-specific functions, without having to duplicate code. Pairing that with meaningful function names increases readability and comprehension of what’s happening where.

Examples: a function for doing some specific debug-logging logic, called by multiple events. Or a function for parsing/cleaning input data. Or a function for writing specific db updates that happen in multiple scenarios.

Event-Specific Update Methods

Problem: If in one Observer class you attach to a number of different events, a single update function will fire for all of those events … but that means it will likely be receiving different function parameters depending on which event was triggered.

Solution: Your observer’s update method can be split out into multiple functions that are customized based on the notification received, thus also easily interpreting the passed parameters coming from where that notification was triggered.

So rather than use one update method, create separate methods to receive each type of event, as follows.

There are two syntaxes supported for this. The update method will only be called if neither of these other syntaxes are found:

snake_cased event name

This is supported since v1.5.7.

If the notifier begins with NOTIFY_ or NOTIFIER_ then your observer class’s function can use the same name.

Example: For a notifier named NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR you could have a function in your observer class named notify_product_views_hit_incrementor() instead of update() (and still specify all the same parameters as you would for an update() function.

This has the added benefit of being searchable just like the notifier event name (albeit lowercase).

Alternatively, or for older versions, use the camelCased convention described below:

camelCased event name

With this alternative syntax, the update method’s name consists of the word update followed by the CamelCased version of the watched-for notification. For instance, the Products Viewed Counter described above could be recoded as an auto-loading, customized-method observer:

class zcObserverProductsViewedCounter extends base 
    function __construct() 
        $this->attach($this, array('NOTIFY_PRODUCT_VIEWS_HIT_INCREMENTOR'));

    function updateNotifyProductViewsHitIncrementor(&$class, $eventID, $products_id)
            global $db;
            $sql = 
                    SET products_viewed = products_viewed+1
                  WHERE products_id = " . (int)$products_id . "
                    AND language_id = " . (int)$_SESSION['languages_id'] . "
                  LIMIT 1";

Generic Formal Parameter Interpretation

If you choose not to use event-specific “update” function names, then in order for your Observer to respond to the varying $parameters received when triggered, you will need to do some special treatment based on the $eventID that was triggered.

The following approach is a way to trick the update method into “interpreting” the parameters using a switch statement according to the specific eventID being passed.

For the procedural-coding thinker, this switch-case approach allows the same code to be used for multiple notifiers if you are also using event-specific methods in the same file.

Here’s an example from the Edit Orders plugin. Each of the monitored events uses different parameters, so the formal parameters used in the update function declaration are generic.

    public function update(&$class, $eventID, $p1, &$p2, &$p3, &$p4, &$p5) 
        switch ($eventID) {
            // -----
            // Triggered during the orders-listing sidebar generation, after the upper button-list has been created.
            // $p1 ... Contains the current $oInfo object, which contains the orders-id.
            // $p2 ... A reference to the current $contents array; the NEXT-TO-LAST element has been updated
            //         with the built-in button list.
                if (is_object($p1)) {
                    $index_to_update = count($p2) - 2;
                    $p2[$index_to_update]['text'] = $this->addEditOrderButton($p1->orders_id, $p2[$index_to_update]['text']);
            // -----
            // Triggered during the orders-listing sidebar generation, after the lower-button-list has been created.
            // $p1 ... Contains the current $oInfo object (could be empty), containing the orders-id.
            // $p2 ... A reference to the current $contents array; the LAST element has been updated
            //         with the built-in button list.
                if (is_object($p1) && count($p2) > 0) {
                    $index_to_update = count($p2) - 1;
                    $p2[$index_to_update]['text'] = $this->addEditOrderButton($p1->orders_id, $p2[$index_to_update]['text']);
            // -----
            // Triggered during the orders-listing generation for each order, gives us a chance to add the icon to
            // quickly edit the associated order.
            // $p1 ... An empty array
            // $p2 ... A reference to the current order's database fields array.
            // $p3 ... A reference to the $show_difference variable, unused by this processing.
            // $p4 ... A reference to the $extra_action_icons variable, which will be augmented with an icon
            //         linking to this order's EO processing.
                $p4 .= $this->createEditOrdersLink($p2['orders_id'], zen_image(DIR_WS_IMAGES . EO_BUTTON_ICON_DETAILS, EO_ICON_DETAILS), EO_ZC156_FA_ICON, false);

Some developers believe this method should not be used since it is less self-documenting than prior methods. Others prefer it for its shorter function name when compared to the more long-winded camelCase syntax.

Additional Information

Event Aliasing

Sometimes notifier names are changed (where triggered in application code) (because of a typo, for example, or to make them more self-documenting). When this happens, rather than just remove the old notifier from the code base, the recommended practice since Zen Cart 1.5.7 has been to alias the old name. This way older code which uses the old notifier name will still work. In some cases it makes perfect sense to fully remove an old notifier call. Use discretion when deciding whether to clean up after yourself or hold on for long-term backward compatibility.

An example of a notifier name which has been aliased due to a typo is NOTIFIY_ORDER_CART_SUBTOTAL_CALCULATE.

Plugins which support Notifier Use

Some plugins which can be helpful during development when using notifiers include:

Information about Notifiers

  • A list of notifiers for the current Zen Cart release.
  • The output of the Zen Cart Notifier Report run against the current release is provided on the docs site for easy reference by developers.
  • You may also install the Notifier Report on your own site to get the list of notifiers for your version, which could differ from the official list if you have made changes or are not running the current version.

Complete Examples

Still have questions? Use the Search box in the upper right, or try the full list of FAQs. If you can't find it there, head over to the Zen Cart support forum and ask there in the appropriate subforum. In your post, please include your Zen Cart and PHP versions, and a link to your site.

Is there an error or omission on this page? Please post to General Questions on the support forum. Or, if you'd like to open a pull request, just review the guidelines and get started. You can even PR right here.
Last modified May 16, 2024 by Scott Wilson (8e3fc43).