WordPress plugins – Front Controller

WordPress supports extensions through plugins. Plugins integrate into the core of WordPress with hooks. These hooks are executed at places in the WordPress code. An example of this is when a web page of posts is displayed, when a user is created or when a post is deleted.

There is an action hook that is called “init” that is executed the moment before the theme gets loaded and posts gets presented. When this hook is executed all plugins are loaded and options from the database are retrieved from the database, and therefore is this hook very suitable for what I am about to show you here.

I often need to call functions available from my plugin from a JavaScript, also known as Ajax, or from an HTML form. As an example I will describe how to create a filter that lets the user use a filter, such that only posts that matches this filter will be shown. I have written a plugin that makes use of this technique. The plugin is not publicly available, but I will show you how it can be done.

This filter sets a cookie in the users web browser. The user will then have the filter active until the users changes to some other filter. I will show you two examples, first where I use a regular HTML link to change to a different filter, then I will show you how Ajax can be used to retrieve a list of available filters. Filters can be placed in levels relative to each other, where a filter will have a parent filter. Think of it as categories in WordPress.

The link to change a filter can look something like this:

<a href="http://example.com/?myplugin=set&filter=sport">Sport</a>

When i click on this link I need my plugin to check for those parameters I send. In my examples I call my plugin for “myplugin”, although thats not the real name of it, but then again it’s only an example ;) . In the URL in the example above I have used the code word “myplugin”, the same as the name of the plugin. This is to reduce the possibility of name collisions with other plugins. In the PHP code of the plugin I have connected a function to the “init” hook that checks if the code word has been given as a parameter. It will then look for a suitable action to execute based on the value of the code word.

The following example shows the PHP code that accepts both a request to set a filter and to return a list of all filters that has a certain parent. If the first test matches a filter gets set and WordPress continues to load the page. In the second test you can see that the script dies the moment after it prints the list of requested filters. That is actually desired because we are going to make an asynchronous JavaScript call to this function that will want only this list, not a normal WordPress page. With the JavaScript call we are not going to reload the page, but only update a small part of the page, whether it is a <select> list, a list of checkbox’ or some other action decided by the JavaScript.

$myplugin = new MyPlugin();

add_action('init', array(&$myplugin, 'check_action'));


// The main class of the plugin

class MyPlugin {

    function check_action() {

        $action = _REQUEST['myplugin'];

        switch($action) {

            case 'set':



            case 'list':

                $list = $this->get_list($_REQUEST['parent']);

                echo $list;




    // Set a new filter

    function set_filter($filter = '') {

        // use setcookie to set the filter in the web browsers cookie


    // Returns a list of filters with a given parent

    function get_list($parent = '') {

        // lookup all the filters with the parent given as a parameter



I will not go into details of how JavaScript can be used to make asynchronous calls. It would be too big and too many details to dig into in this article. There are many good resources on the Internet about Ajax, and Google knows about most of them. It can be a good thing to check out Cross-Browser XMLHttpRequest that makes it a lot easier to use Ajax without all the pain of different techniques and implementations between web browsers. Prototype and jQuery are two very powerful JavaScript libraries that are worth to have a look at as well. Beware that those two are per default not compatible with each other, use either Prototype or jQuery if you are not already familiar with them.

This article is a translation of my norwegian article from march 8.

A new category structure in WordPress

The proposal

I personally don’t like the way wordpress presents categories, with URLs such as “http://example.tld/category/some-category”, and especially not the possiblity to put a post inside several categories. That’s a job for tags. Therefore I suggests a new category structure where we use a page as the parent (or category) of posts.

URL hierarchy

Pages gets a URL like “http://example.tld/my-page”. By using a page as the category of our post we can have a URL like “http://example.tld/my-page/my-post” or “http://example.tld/my-page/2007/04/14/my-post” for our posts. As a bonus the “category” page would work as SEO Siloing.

New page template

This would also bring up the need for a new page template similar to the archive template, where the latest posts inside the category/page can be displayed together with the content of the page. An example would be that the five last posts in that category/page could be listed beneath the page content. This would replace the category templates where the file name of the template files ends with the ID of the category, such as “category-14.php”, which is much harder to maintain on a page with many categories.

Post templates

With this solution the posts will have only one category, the page it was assigned. That way it is possible to have post templates based on the nicename of the page. This will solve the problem that I have today when I need different layout of my posts based on which category it belongs to.

Default page for posts

The default category for posts will then (of course) be the front page, and a post on the front page will get a URL as “http://example.tld/my-post”.

Database structure

Posts and pages uses the same database table ($wpdb->prefix . ‘posts’) and we then need a link between a post and a page to make this relation possible. My purpose would be to create a new table ($wpdb->prefix . ‘post2page’) to handle the relation between posts and pages where we will have two columns, one primary key that would be the “post_id” and the other would be the “page_id”, both referring to the same posts table and the column “ID”. This is because I want to leave the posts table as is and as well making it easier to extend this relation later on.

Category management

There would then be no need for the categories as we know it from todays WordPress. Tags, that will come with the WordPress 2.2 release, will be able to replace the functionality of adding a post to multiple categories.

I wrote this as a ticket on WordPress Trac.

Update: 15.04.2007

A category can have subcategories to make up a hierarchy, but pages can also be ordered like that. A page can be a subpage of another page.

It might be desirable to have pages that are not behaving as a category for posts. This functionality can be achieved by adding a checkbox to control this the same way comments and pings can be turned on and off on a page. Alternatively the author of the page can select a template that doesn’t display any posts related with the page.

There are users that insist on placing a post inside several categories. A solution to this will then be to allow for multiple posts-to-pages associations by using a combined primary key in the post2page table over both the “post_id” and “page_id” columns.

Update 15.04.2007 (2)

I know it is a bit radical thinking and a big change to WordPress by doing this, but you might want to write posts that are related to your page. For example if you write a page with information about WordPress and your plugin development, then you probably want to publish your new plugins as posts, not as subpages of the WordPress page. The exception from this is pages such as “about”, “legal” and so on, but with the ability to disable posting to those certain pages (see above) this makes it possible.

It was just a suggestion to improve WordPress. It is not a specific issue I’m having any longer, since I managed to give posts in different categories the diffent look I needed by placing both the logic and the view/html inside a plugin. A bad solution, I know, but it works. I didn’t want too much logic in the theme files that are dependent on some plugin. The different looks I needed was not only when displaying the posts on the category page, but also when using the “single” template.


Welcome to Datafeel.

My name is Knut-Olav Hoven and I am a software developer at a company named Linpro. You can find my (more) personal web site at http://www.hoven.ws.

Datafeel is my own registered company with zero income and zero expenses. I actually wonder why I registered it, I guess I just thought “Datafeel” was a nite name for a company :-) .

I recently found a good use for this web site, to use it as a base for my software and techniques I want to share with the rest of the world. To begin with I started to write some information about WordPress and will be sharing my plugins.