How to build a WordPress plugin for beginners

The first steps to building your own WordPress plugin

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WordPress is the most popular blogging and CMS platform out there today. Part of what has made it so successful is its ability to be extended via plugins to perform any function you desire. Developing a plugin framework is really difficult, but WordPress has one of the best solutions I’ve seen to date. So how do you go about taking advantage of it and building your own plugin?

WordPress development is done in PHP, which makes it pretty accessible as developers generally know their way around the language. Building a plugin is just a matter of creating a basic file structure and using WordPress provided functions and event hooks to drive your application.

The File Structure

All it takes to create a plugin is a single PHP file, but common practice dictates that you house your plugin and its files within a directory. To begin, create a directory to hold your plugin and give it a name matching your plugin. Then, within that directory, create your main PHP file, again naming the file with the name of your plugin:

+ super-plugin
    - super-plugin.php

If your plugin is going to require any images, javascript, css, or additional PHP files, you will house those within this directory as well:

+ super-plugin
    + admin
         - super-plugin-admin.php
    + img
         - icon.png
    + js
         - super-plugin.js
- super-plugin.php
- readme.txt

In this example, I’ve created 3 directories, admin, img, and js to hold my additional files. The super-plugin-admin.php file will be used to provide a web interface to my plugin on the WP backend which I’ll address in another post.

Finally, if you plan on hosting your plugin in the WordPress plugin directory, you’ll need to include a file named readme.txt at the root of the directory. The file should follow the format detailed in this readme.txt example.

Defining Your Plugin

Next, we need to define our plugin so that WP will recognize it and allow it to be installed, removed, and activated.

Open your main plugin file, super-plugin.php and add the following to the top of the file:

<?php
/*
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
License: A "Slug" license name e.g. GPL2
*/
?>

Replace each line with the data for your plugin. This will define your plugin details for WordPress. At a minimum you need the Plugin Name: field for the plugin to be recognized.

Initializing your program

Now that WordPress knows about your plugin, it’s time to make it do something. This is accomplished by using hooks and predefined WordPress functions. A hook is an event listener that is trigger based on outside events occurring. If your program needs to perform any type of setup, such as database table creation, you can tap into the plugin activation hook like so:

//SETUP
function super_plugin_install(){
    //Do some installation work
}
register_activation_hook(__FILE__,'super_plugin_install');

Then, when a user activates your plugin, any function that has been registered with the activation hook will be executed.

You might also want to register some custom javascript that your plugin will need to operate. You can do this by using the add_action hook to piggyback on to another process. In this case, we’re going to listen for the wp_enqueue_scripts event to trigger and let WordPress know that we want in on the script party and to please execute our function as well.

//SCRIPTS
function super_plugin_scripts(){
    wp_register_script('super_plugin_script',plugin_dir_url( __FILE__ ).'js/super-plugin.js');
    wp_enqueue_script('super_plugin_script');
}
add_action('wp_enqueue_scripts','super_plugin_scripts');

When scripts are being loaded up, your function will get executed and your script will get registered and queued to be added into the head portion of the HTML.

Finally, you want to actually run your program. You can do this by listening to various WP hooks and waiting for an appropriate event that would trigger your code to run. If you want to run your program every time a visitor comes to your site, you can use the ‘init’ or ‘wp_loaded’ action to trigger your code:

//HOOKS
add_action('init','super_plugin_init');
/********************************************************/
/* FUNCTIONS
********************************************************/
function super_plugin_init(){
    //do work
    run_sub_process();
}

function run_sub_process(){
    //more work
}

In this example, the super_plugin_init() function will get called on every new request and you can perform whatever logic you need. If your plugin is complex, you can add additional PHP files to build out your application and just use this initial hook to trigger the full application. A full list of the actions that run during a typical request can be found here.

Conclusion

With these elements in place, you’re off and running with your own WordPress plugin. What you do with it is up to you, but you’re only limited by your imagination. Uploading your directory to your WordPress installation will make your plugin appear in the plugins section and you can activate it like any other plugin.

Next up, you’ll probably want some way to administer the plugin on the backend of WordPress. In my next post, I’ll show you how to get started with that process.

Read more of Matthew Mombrea's ByteStream blog and follow Matt on Twitter (@mombrea) and Google+. For the latest IT news, analysis and how-tos, follow ITworld on Twitter and Facebook.

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