Tutorial: How to write a WordPress Plugin?

WordPress is not just a blogging platform and it is such a powerful CMS with unlimited capabilities, besides having a huge user base. Almost anything can be scripted with wordpress. You can extend wordpress either by means of plugin or by a theme.

In this tutorial, i will show you how to write a Hello World wordpress plugin, which unlike many believe is surprisingly easy, once you understand the very fundamentals. All you need to have is a basic knowledge of php scripting.

Before we move on coding a plugin, please make sure you remember the following coding practices.

1. Always you chose a unique name to your plugin so that it doesnt collide with names used in other plugins.

2. Make sure you comment wherever and whenever necessary in the code.

3. You will to test the plugin in your localhost (using xampp) along with latest version of wordpress.

Plugin Files & Names

Assigning unique names, documenting and organizing the plugin files is very important part of plugin creation.

Although wordpress allows you to place the plugin php file directly into the wp-content/plugins folder, for a good plugin developer you will need to create a folder named hello-world and within place readme.txt and hello-world.php.

The readme.txt contains information about your plugin and can come in handy when you submit your plugin wordpress SVN plugin repository. See the readme sample.

Go ahead and create these files and we will add the content to these files later.

The Plugin Basics

The heart of a wordpress plugins is the below 2 functions (commonly called `hooks`)

add_action ($tag, $func) documentation
add_filter ($tag,$func) documentation

It is very important to know the difference between the above functions.

  • add_action –> does an action at various points of wordpress execution
  • add_filter –> does filtering the data (eg. escaping quotes before mysql insert, or during output to browser.

Refer to the WordPress Plugin API for more better understanding.

Plugin Information

Open your hello-world.php and in the first line, add this commented plugin information to your file.

<?php
/*
Plugin Name: Hello-World
Plugin URI: http://yourdomain.com/
Description: A simple hello world wordpress plugin
Version: 1.0
Author: Balakrishnan
Author URI: http://yourdomain.com
License: GPL
*/
?>

Save this php file,

  • Place the plugin folder to wordpress > wp-content > plugins,
  • Go to your wordpress admin > plugins and you will see the new plugin listed, waiting to get activated.

simple ain’t it?

But this plugin had to do something right?

Why not we make it print  “Hello World” when we call it from wordpress theme template files.

for that we write the code using add_action below the commented plugin information in the hello-world.php

<?php
/*
Plugin Name: Hello-World
Plugin URI: http://yourdomain.com/
Description: A simple hello world wordpress plugin
Version: 1.0
Author: Balakrishnan
Author URI: http://yourdomain.com
License: GPL
*/
/* This calls hello_world() function when wordpress initializes.*/
/* Note that the hello_world doesnt have brackets.
add_action('init','hello_world');
function hello_world()
{
echo "Hello World";
}
?>

Thats it! Our Hello World plugin is nearly done and with just few lines of code. When our plugin is activated, add_action command calls our hello_world() function when wordpress starts loading.

Lets Test our Hello World Plugin

We really dont know whether our plugin works or not. To test our plugin, go to plugins, activate the hello-world plugin.

Then open your worldpress theme wp-content > themes > default, open any of index.php, archive.php or single.php and place the following code anywhere.

<?php
if(function_exists('hello_world')) {
hello_world();
}
?>

The key here is function_exists() call which checks whether plugin loaded or not and then allows the hook into the plugin function.  Call to hello_world() in the theme files without checking it, often leads to “Fatal error: call to undefined function” and our blog would crash, if the hello world plugin is not activated or deleted.

If you carefully notice above graphic, see how the hello world appears. Thats the work of our plugin. It WORKS!

Lets take our plugin one step further!

Why not, we build a plugin options page in admin area and provide a backend for plugin users?

Right now, the plugin outputs hello world (its pretty much static) and if somebody wants to output ‘Hello Example’, they need to open the php file and make changes everytime to print different text.

Asking the user to edit plugin files isnt a good idea!  As a wordpress plugin developer, it is you, who has to provide a good wordpress options interface in the wp-admin area.

Writing Plugin Options Page

We now create  Hello World options page in the wordpress admin area.

Here is what we do….

  • When plugin gets activated, we create new database field `wp_hello_world_data` using set_options() function.
  • When plugin gets deactivated, we delete the database field `wp_hello_world_data`
  • We create a options menu for Hello World in WordPress Admin > Settings.
  • We save the user entered data in the wordpress database.
  • We retrieve the data stored in wordpress database and output it using get_options() function.

Why we are creating database field? because the saved data must be saved somewhere? ie. in wordpress database. This way the plugin outputs user entered text, instead of the static “Hello World”.

Activating/Deactivating Plugin

It is very easy to write a function on what plugin does, when it gets activated. WordPress offers 4 very important functions

<?php
/* Runs when plugin is activated */
register_activation_hook(__FILE__,'hello_world_install'); 

/* Runs on plugin deactivation*/
register_deactivation_hook( __FILE__, 'hello_world_remove' );

function hello_world_install() {
/* Creates new database field */
add_option("hello_world_data", 'Default', '', 'yes');
}

function hello_world_remove() {
/* Deletes the database field */
delete_option('hello_world_data');
}

?>

The above code creates the new database field `hello_world_data` using add_options and it runs when we activate the plugin. It has the value ‘default’ since we explicitly define it.

Similarly when the plugin gets deactivated or removed, we need to clean up things, so we remove the created database field using delete_option.

Plugin Settings Page

This is our final step.  All we need to create is plugin settings page in the wordpress admin area.  The settings page will update and save the data to the database field `hello_world_data` which we created while activating the plugin. Be sure to checkout creating options page in wordpress codex.

Here is a very important thing to remember:

The add_action for admin_menu should call a function hello_world_admin_menu() containing add_options_page, which in turn should call a function hello_world_html_code() containing html code. This is how the code should flow! Refer to wordpress administration menus

<?php
if ( is_admin() ){

/* Call the html code */
add_action('admin_menu', 'hello_world_admin_menu');

function hello_world_admin_menu() {
add_options_page('Hello World', 'Hello World', 'administrator',
'hello-world', 'hello_world_html_page');
}
}
?>

The above code, is placed under is_admin() which means it only runs in the wordpress admin area.

The below function has the html code for the settings page, containing the form and notice how the php tag is split to accomodate the html code.

and the coding part is..

<?php
function hello_world_html_page() {
?>
<div>
<h2>Hello World Options</h2>

<form method="post" action="options.php">
<?php wp_nonce_field('update-options'); ?>

<table width="510">
<tr valign="top">
<th width="92" scope="row">Enter Text</th>
<td width="406">
<input name="hello_world_data" type="text" id="hello_world_data"
value="<?php echo get_option('hello_world_data'); ?>" />
(ex. Hello World)</td>
</tr>
</table>

<input type="hidden" name="action" value="update" />
<input type="hidden" name="page_options" value="hello_world_data" />

<p>
<input type="submit" value="<?php _e('Save Changes') ?>" />
</p>

</form>
</div>
<?php
}
?>

You must remember 2 things in the above code.

1. Specify the database field we created before in the input text box as `hello_world_data`

<input name="hello_world_data" type="text" id="hello_world_data"
 value="<?php echo get_option('hello_world_data'); ?>" />

2. If your form has number of fields (like textbox, selectbox etc), all those should be listed in the value field of page_options, separated by commas. For more information, refer to wordpress documentation

<input type="hidden" name="page_options" value="hello_world_data" />
 Now, the plugin outputs whatever the user specifies in the hello world settings page.

Thats it! Our plugin is READY!

Dont forget to add documentation to readme.txt.

Enjoy!
___________________________________________

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Balakrishnan Prabhu

Mr. Balakrishnan Prabhu is the founder of Corpocrat magazine. He is also the founder of Best Citizenships (BC), the world's leading firm, assisting very wealthy individuals and investors with Global citizenship solutions. His other interests are Linux servers, Machine learning, Parallel computing, Computer vision, Wordpress, etc. You can contact him here

  • Uday Augustin

    Thanks a lot. Great Tutorial. Very easy to understand.

  • Drum

    Hi,

    I am able to display the form in admin area…
    But it isn’t getting saved in database…
    and html form contains options.php..what would be the content of options.php and where it should be saved….

    “<input name="hello_world_data" type="text" id="hello_world_data"
    value="” /> ”

    I want to confirm if textbox will contain the value already in database or it will be always blank..

  • http://www.fpmgonline.com David

    AWESOME! This is exactly what I was searching for & after hours of searching, I finally found an excellent tutorial that makes sense. Thanks for the share PBU.

  • Ashit Kumar

    This is a very good for beginner to start in wordpress plugin development.

  • Lohith

    simple to learn, Easy to implement thank you:-}

  • http://sandeep-pattanaik.blogspot.com Sandeep pattanaik

    Nice to read your post. I am new to plugin development.

  • Rahul

    great tutorial boss and nice write. You have just missed mentioning about option.php where the forms content are saved and updated otherwise it would have been a best example.

  • shalu

    really , it helps me…….thanx

  • http://www.knoje.com Knoje

    Really Helpful,
    I was searching for such tutorial and its the best one.

  • http://mcamaster.blogspot.com AMIT DAVE

    Respectede sir

    I am Creating as you say the first plugin but when i create the second one there are trouble. after creating my plugin how can i put to my web pages. or where i put it . for ex : first plugin you say that put it in index.php one function . thats this second plugins how can i put ??

  • hello

    I like this tutorial.

  • http://sizzledenignz.com Gagan Garg

    Hi mate,
    This is awesome code.It really helps me for creating the plugin’s.I’m very thankful to You

    Thanks again.

    (Gagan Garg)

  • http://phpwebbies.com/ Anir

    Thanks for sharing! Very helpful tip, very informative.
    This is a great help for an amateur like me.

  • Ammar Ahmed

    What a great and simple tutorial, although i dont understand much of HTML but i do understand the concept now. Thanks a lot man.

  • James

    Why the text we input show in header? how to change to other place ? plz can you tell me ?

  • http://www.onlybusinesstalks.com/ Hemant

    Hi Prabhu,

    Your article is really helpful, thanks for writing. I am looking forward to read more articles in future. Keep writing!!! cheers.

  • s srinivasareddy

    hi sir
    this is srinivasareddy from india

    i need more information on this

  • s srinivasareddy

    your article is really good

    i got more information

  • gulnaz

    this is sooooooo helpful artical thanx.

  • murugan

    A very good help for beginner thanks.

  • http://davidsteenkamp.com David Steenkamp

    Can you clarify exactly what files should be created at the end? According to this tutorial you only need to have hello-world.php and readme.txt in the hello-world folder that is uploaded to the plugins directory. I think some clarification would help everyone! Thanks.

  • http://no smruti

    this is very good tutorial for a begineer.
    thank u so much

  • http://anthonypritchard.co.uk Anthony Pritchard

    Very nice, concise tutorial. Just what I’ve been looking for. Good work. thanks!

  • Monirul

    I would like to check with how I can a plugin for affiliate website. I want to build an affiliate website to sell products online and track that.

  • junkrchelidk

    This was a great helps thanks.

  • nikunj vaghasiya

    Your article is really helpful, thanks for writing. I am looking forward to read more articles in future. Keep writing!!!
    Thanks for sharing

  • Dave

    As someone who did not knew ANYTHING, that helped me so much .. thanks !!