Secrets of a WordPress plugin

A while ago I wrote a small tutorial on how a WordPress theme is created, now I would like to share some “secrets” of WordPress plugin programming, so here we go
.

So what kind of plugin are we going to create? Let’s do a plugin that gets some WordPress data from the database and puts it on a static page. shall we.

The first thing we need to do is create a plugin file with the correct information in the header so that WordPress can identify it as a plugin. We will call this plugin wp-plugin. The normal thing to do is give the plugin file the same name as the plugin itself, hence the name for our file is wp-plugin.php
. We put this file in the wp-content/plugins directory of our WordPress installation .

It’s all in the head

The header of wp-plugin.php contains information about plugin name, author name and author email, web site an so. The most important is the plugin name
.

wp-plugin.php

When the user visits the WordPress admin->plugins WordPress scans the plugins directory for plugin files so if the header is correct the file is added to the interface and can be activated or deactivated.

Lets take a look at the code. As you can see we have specified some meta information of the plugin, the code is self explanatory and not hard to understand. This is what WordPress uses to mark the file as a plugin as long as it’s located in the correct folder.

Hooks and filters

For the plugin to interact with WordPress it need some API to interact with so WordPress has it’s own API for plugins.

Hooks are provided by WordPress to allow your plugin to ‘hook into’ the rest of WordPress. There are two kinds of hooks:
Hooks that take Actions. Actions are functions that specify additional events that can occur triggered by core WordPress events. Action Hooks, then, enable the addition and removal of functions whose execution will be triggered by those WordPress actions. These hooks enable plugins to execute functions each time the specified event occurs.

Hooks that take Filters. Filters are functions that modify WordPress content stored in your database
. Filter Hooks allow you to add or remove Filters to various types of WordPress content (usually text). These enable plugins to modify content or text on-the-fly.

From a coding perspective, actions and filters are nearly identical. The add_action function is nothing more than an alias for the add_filter function. In practice, they are useful for different things.

You can sometimes accomplish the same goal with either an action or a filter. For example, adding an action to publish_post or a filter to the_content. Choose based on whether you want the change to be made once or to be made every time the content is accessed.

So lets write some code

|", $url, $content);
}

/**
*   This will be added to the header of WordPress trough the
*   add_filter below.
*/
function my_header()
{
   
}

/**
*   Install filter and hooks.
*   WordPress sends the content from the
*   database to the my_function above 
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.
*/
add_filter('the_content', 'my_content');

/**
*   This will be added to the header of the WordPress 
*   before it is displayed, perfect for adding your own css or scripts.
*/
add_action('wp_head','my_header')

Let’s take a look at the code above. At the end we define our hooks and filters that interact with the API. The function my_content($content) is where it all happens. This function takes the content generated from the database such as the content of a page or a post. This is done via the API and WordPress is responsible for the content that is send to the function. In this function we retrive some trivial data from the database and puts it in a variable.

Now the difficult part comes. We have to replace our tag in the static page with our data from the database, and that is done with preg_replace, we simply replace our whith the content in our variable. Last we returns the content back to WordPress so it can be displayed.

For this to work we need to create a static page and somewhere we put our tag to replace.

Summary

• Create a file
• Add the header
• Add hooks and filter
• Create the functions
• Replace data and return the content

Further reading

WordPress Codex